Cured epoxy molecule
|Cured polyester molecule|
The other primary use is coating. If you elect to coat parts, you have to go the full nine yards. We call this encapsulation and it's just as the word suggests, you embalm the part in goo (I call epoxy a number of things depending on my mood, goo is a favorite). To be effective, typically as a moisture barrier, every square inch of surface, including screw holes, cutouts, notches and especially end grain has to be well coated with goo. I recommend at least three coats on every surface. Three coats generally insures everything has a thick enough coating, to be waterproof.
What this procedure (encapsulation) does is stabilize the moisture content within the wood, at the level it was when the epoxy went down. With the wood's moisture content “locked down”, you've eliminated any possibility for rot to set in. It also prevents moisture soaking into the wood, water logging it and adding weight. The wood should be 15% or less moisture content. Most manufactures prefer less; around 12% is a common recommendation. Stabilized wood doesn't swell and contract with environmental changes (humidity). This is encapsulation's biggest benefit.
Stable wood doesn't try to pull apart glue joints, from swelling or contraction. Fasteners can't be stretched or yanked out of their holes, when moisture gain or lose doesn't move the wood. This is a huge advantage from a structural viewpoint. A homogenous assembly of pieces results, literally a single, glued unit that can't be worked apart, without physically destroying large sections. Once engineers understood this, whole new building methods reared up, producing very light, strong, waterproof structures. Very often, these structures have none of the traditional elements found in wooden boats. Boats with no frames, no metal fasteners, no structural floors, no stringers, not even a keel, have been designed that are just as strong and very likely lighter than their traditional build method brothers. As I said, a huge advantage.
A lot has been mentioned about the perils of epoxy use. Most of the horror stories have come from folks that have become “sensitized” to the stuff. Many of these used improper technique and methods, causing direct contact or inhalation, which understandably has caused them issues. A small percentage of people have worked clean, yet still developed an allergic reaction. Those that have difficulty with particularly alkaline substances will probably have problems with epoxy. If you are one of these, take special precautions to insure you don't breathe sanding dust or smear epoxy on your skin.
Generally, you don't want to eat it. It's not very tasty anyway, so don't trouble yourself. Keep it off your skin. Don't breathe the dust when sanding cured epoxy. Epoxy is most dangerous when absorbed through the skin and when inhaling dust. Dust is a rock hard particulate and pokes little tiny holes in your lungs if it isn't gathering into little clumps on mucus membranes. This affects you much like coal miners “black lung”. It will eventually cause, with enough exposure, troubles like COPD, lung cancer and all of the inhaled difficulties you can imagine.
A full epoxy cure can take a couple of weeks in normal temperatures and longer if it's in cooler conditions. The coating seems cured, is hard enough to sand and shape, but it's technically still chemically "active". This means if you sand it, the pesky particulates not only get into your lungs, but being embedded in mucous membranes, transmit these still active chemicals, directly to the blood stream. Being in the lungs means, the next stop (for these chemicals), once in the bloodstream is your brain, so not good. Simple point, if the epoxy you're about to sand is less than a couple weeks old, it may be quite dry, but it's still chemically active, so use a good particulate mask. Some like a full up respirator, but this isn't necessary for the casual user. Now, if you're making a living working with goo, yeah, think about the long term need for more than a dust mask. Now that I've scared the crap out of you, once epoxy is cured, it's chemically inert. It's so inert, the things that get implanted into humans (pacemakers and other devices), have circuit boards and wiring, typically sealed from body fluids with (wait for it) epoxy. That's pretty damn inert.
Liquid goo soaks into the skin and gets into the bloodstream. Using solvents like acetone, denatured alcohol, MEK, etc. will just make this much, much worse, so don't do it. Use white vinegar, non-pumice hand cleaner (like GoJo) or an acidic liquid with a slow evaporation rate. I touched some wet epoxy and was drinking a bottle of orange juice once. I looked at the OJ and realized it was probably as good as anything else was. I poured a little on the spot, rubbed my hands together and sure enough, it cut the epoxy. Acidic liquids will stop the reaction of the epoxy cure. The more powerful the acid, the more it acts as a cutter. This said, a powerful acid (typically solvents) will also help dissolve and/or dilute the epoxy, so it can transmit through your skin better/faster.
Use paper towels to mop up the bulk of the goo first, then onto chemicals. Clean up tools with white vinegar as best as you can, then use a little acetone to finish the job. This is the most economical way to handle tools.
Treat hands much like tools, except for the acetone. Paper towel the bulk of it off, then use white vinegar to get the rest. Follow up with a good hand cleaner. I like the ones that are citrus based. They're natural, biodegradable and you don't smell like a pickle afterward, plus they don't cost much.
I've worked with epoxy for 30 years and don't have much sensitivity to it. I do have a very slight sensitivity to some brands (formulations), so I just don't use these brands. In the early years of my efforts, I was fairly casual about protection, but now am pretty anal about it, mostly because I discovered I was sensitive to this particular brand. It was this that caused me to study up on the chemistry and learn about the various formulations (over a dozen different room temperature cure versions alone). I'm currently testing a new(ish) mixture, for a large industry formulator. We'll see how this stuff ranks against the others, but it's showing some promise so far.
Everyone I know that does a fair bit of work with epoxy has developed a “goo station”. This dedicated area in the shop or workspace, used solely for storing, mixing and cleaning up epoxy. My epoxy station includes all the chemicals I use in the shop; paper towels, gloves, mixing cups, brushes, rollers, roller trays, tray liners, containers of fillers, rolls or boxes of fabrics, 'glass tapes, cleaners, a sink with running water, epoxy resins, hardeners, mixing sticks, spreaders, trowels, plastic sheeting, Mylar sheeting, masking tape, plastic packaging tape. I'm sure I missed something in this list, but you should get the idea.
This station is where I can find any chemical I use, clean up any mess I can make, other then dust or wood chips and of course handle epoxy cleanly and safely. I can also wash my hands or clean up a part in the sink. My sink is one of those plastic utility tubs seen in any laundry room.
Directly next to this station, I keep my paints, which is handy since the solvents for these are at the epoxy station. I also keep all oils in the paint section. My shop serves many functions, so motor oil, outdrive gear case oil, 2 stroke oil, linseed and tung oil, hypoid gear oil, penetrating oil, axle grease, etc. are also stored here. This keeps everything in one general area of the shop and since it's the most likely place for a nasty fire, I've located it at the front corner of the shop with a fire extinguisher near by. If it does catch fire, I might lose the corner of the shop, but not the whole thing and the chemicals are contained in this single location, so unexpected explosions or secondary fires are less likely.
My epoxy workstation has a flat Formica covered counter space (just an old kitchen countertop), where I do the dispensing, mixing and clean up. Hanging on the wall are a few different size pieces of 1/4” plywood. These serve as mixing trays. Covered with several coats of neat epoxy, I do my final mixing with fillers on these boards, if the thickened mixture will be relatively stiff. For sloppy or runny mixtures, I use a few different sizes of Tupperware cake carrier bottoms. I found these at the local discount department store. They cost a couple of bucks; the largest is about 12” x 18” and 3” deep. The bottom is flat, without bumps or ridges and the corners have a big radius, so they can be cleaned and epoxy scraped out easily. Being PVC, the cured goo pops out easily.
I use these boards or cake trays to spread out the thickened mixture into a very thin sheet. This keeps epoxy from kicking off prematurely. Massed epoxy, in a lump or at the bottom of a mixing cup, will quickly build up heat and cure. It can actually build up enough heat to catch a paper cup on fire, so spread out your mixtures as soon as you get it mixed folks - it'll stay workable for much longer.
Having everything at the ready is one of the keys to successful epoxy work. When the gloves are on, covered in goo, you don't need to be opening cupboards looking for a putty knife. You'll have globs of goo on everything in short order, so get organized and have everything in a handy location.
Epoxy brands intended for the marine markets are all about the same in regard to physical attributes. The major formulators are costly, though their products are reliable. There are several discount formulators, most sell online and generally at 1/2 to 1/3rd the price of the big guys. Do some looking around and you'll quickly find in smaller quantities $60 - $65 per gallon pricing. If you need more than a few gallons of goo, buy it in bulk, as prices drop significantly. I'm getting it around $45 a gallon, but I also buy buckets of the stuff.
Epoxy comes in various hardener/resin ratios. Some are 1:1, but most aren't. Mixing pumps can solve this problem and for the most part, they work fine. If you're using a brand that doesn't offer dispensing pumps, then you'll have to resort to volume measurements. The easiest way to do this is have two identical, transparent cups and pre-marked stick. Mark the stick by pouring water into a cup in one ounce intervals, marking the stick at each. If your epoxy mix ratio is 2:1 (for example) then multiples of two into a cup, for every single tick mark in the other cup. Generally, you'll need more resin than hardener, but not with all epoxies, so check the labels. You could mark the sides of the cup, but you have to do this on each, which will lead to errors. Be precise, but don't get anal about it. Epoxy can tolerate being off a little bit (a few percent one way or the other). The higher the resin/hardener ratio, the less true this is.
With resin in one cup and hardener in the other, pour both into your mixing container. In most cases for me, this means the plastic cake carrier. This allows me to mix, but keep the epoxy spread out so it doesn't get hot. I hold the cake tray at about a 15 degree angle and use a plastic applicator to mix the resin. I let it pool along the one edge and I drag the spreader through the epoxy, smearing along the bottom. This smashes the two together for a good mix. It's very important to scrape the bottom and sides of the mixing container. Once you clean out a container that has dried epoxy in it and find a sticky corner, you'll understand why this is important. The sticky corner means not all the resin or hardener got mixed in, so you may have issues with a cure or similar non-cured areas where you just applied the epoxy. Insure to mix every square inch that epoxy has touched inside the container.
If you're adding pigment to the epoxy, do this to the resin only, before you mix with hardener. Mix the pigment into the resin very well, and then add this to the hardener. Pigmented epoxy can even out wood tones, hide imperfections, change the color of wood or if enough coats are applied, act as a solid color base under paint or clear coats. This is something I often do in enclosed spaces within a boat. Typical locations would be inside lockers, underside of decks, inside cabinets, etc. These are places it's difficult to paint, but if you're using epoxy anyway, you can kill two birds with one stone and used pigmented epoxy. I recently did a boat with four cockpit seat lockers, 4 floatation chambers and a forepeak storage area. Painting inside these spaces would prove very difficult. The air chambers will likely never see a paintbrush again, in the life of the boat and with the only access, through a 6” deck plate, extremely difficult without a low yield paint bomb. These areas all received several coats of pigmented epoxy during assembly. This will offer color for the life of the boat, without the need to repaint a hard to reach or difficult area.
Now that you have the resin and hardener mixed well, you can use it as is (neat) or add some filler. If using it neat, then spread it out in a thin coating on the bottom of the mixing tub and take this to the work. I like to use squeegees, rollers and brushes, but there are plenty of ways to apply neat goo. A plastic applicator or squeegee works very well at spreading neat epoxy on a surface. It leaves a uniformly thin coating, which is what you want. With practice, you'll learn how to control the thickness of the film. A notched trowel works well when you need a thick coating (like a tabletop for example). Just spread it around like you're doing tile work in the bathroom. Epoxy's self-leveling properties will fill in the grooves as it sets up. A brush isn't a very economical way to apply epoxy. It takes a lot longer than a squeegee or plastic applicator, it tends to pool rather than spread out and you'll use a lot more epoxy then with a spreader of some sort. Unfortunately, sometimes it's the only thing that will get into corners or other tight spots. A roller works well, but you’ll feel guilty with the amount left in it when it’s setting up.
Adding fillers to the mixed resin takes some practice. It's all about the specific task as to what type of filler and the consistency you need. Let's say you need a structural fillet, which will be taped over. This doesn't require it be especially smooth, just strong and with good bonding qualities. A 50/50 mix of silica and filled 'glass fibers will work. Add the fibers first, a little at a time, mix completely. Next, add the silica, which takes some practice as it floats and doesn't want to cooperate, because it's so fluffy. The easiest way is to smear it into the epoxy. I drag the epoxy out, into a thin sheet on the bottom of the container and then try to smash the silica into it on the next swipe. Other materials are difficult to mix too, but silica seems to be the prick of the bunch. Once both are mixed and sufficient filler added to offer the constancy you need, spread it out, thin on the bottom of the container and take it to the work. Try a stiffer mix (more silica) for vertical and overhead work so it won't sag or run.
When mixing filler materials, I use a technique I saw a printing press operator use to mix ink. Pool the goo in the center of your mixing board or tub then spread it out in a thin sheet. Scrape it up onto your plastic applicator in the next swipe and fold it back on itself, continuing to smear it out in a thin sheet, in the other direction. This scraping and folding action is very effective and quick.
Filling Holes with Goo
Temporary fastener holes, covering screw and nail heads, fixing dings when you dropped a hammer on the bottom planks, etc. all need filling.
There are two types of hole filling, structural and cosmetic. Structural filling, means the hole will have something bearing on it, maybe it'll be drilled out and used as a bushing (like a centerboard pivot bolt hole) or otherwise needs to continue the integrity of the surface it's in. Cosmetic is just a covering to make things smooth and pretty. Only the filler materials differ .
I use the smash and go technique. Each area to be filled is wetted with neat epoxy first, if it's raw wood. If it's had an epoxy coating, I just scuff it up with some 80 grit to offer some “tooth”. With a relatively stiff mixture of epoxy mixed, I use a plastic applicator or flimsy putty knife and mash a glob of goo into the hole. I then smear it flush, but don't attempt to make it perfect. In fact, I try to insure it has a very slight depression at the hole, which I fill later. I do try to keep the area around the hole clean and free of built up goo, which saves fairing work later. When the filled hole has cured, I go back and use a lightweight fairing mix to level the hole, flush with the surround surface.
I developed this technique after trying to do a single step version with limited success. If you have a tough mixture, standing proud of a hole and you try to sand it, you'll just sand away more wood then epoxy, making fairing a lot more difficult. If you assume this will happen and fill the hole slightly shallow, you can then come back with an easy to sand filler, top off the hole’s dimple and quickly knock down this mixture flush.
Don't bother filling holes as you go, you'll drive yourself nuts. Wait until you have a bunch of them and then mix up a batch to do them all. The same is true of topping them off with the lightweight mix. I usually break them up into groups, depending on what type of filling I'll be doing. This way you can walk around the boat looking for shallow dimples to hit or smash and go with the bigger holes.
Filling Seams, Divots and Dings
Seams, dings, scratches and other surface imperfections are filled much like a screw hole or fastener head. An initial coat of thickened epoxy is smeared over the offending area. I try to keep the applicator nearly vertical and in contact with the surrounding surface. This scrapes the epoxy off these surrounding surfaces, leaving only goo in the blemish. You will save a good bit of extra sanding, if you try to apply epoxy only to those areas you need it. The mix suits the job, thicker for things that need to “stand up” thinner if the work is flat.
Seams are slightly different. There are two sides to a seam, usually the inside is filleted and/or taped. The outside receives a filler coat on a rounded over joint area. Usually, the exterior has tape applied over the seam, so the epoxy just serves to fill the gaps and bond the edges together. The outside of a seam may also eventually get a cloth sheathing, so efforts are made to make it smooth, pretty and fair.
I use plastic applicators or purpose made fillet sticks to form fillets. On a hull seam, such as the chine or centerline, I make generous fillets with an applicator, but on cosmetic seams, like those used inside the cockpit to help shed water, I usually employ a stick or section of PVC pipe. The pipe helps keep them uniform in size so they look good. A plastic ball works very well at making nice, even fillets, too.
Apply cloth directly over the fillet once you’ve formed them. They’ll be wet and this is good for the cloth. I use a squeegee or plastic applicator to spread out the tape and continue wetting out the cloth. Don’t try to smooth things out too much, you can touch up with filler later. The idea is to get a good shaped fillet covered with tape, without disturbing the fillet too much. Often it’s better to wait for the fillet to kick off, then apply the tape, though you’ll have to wet the cloth manually, as the tacky fillet will not be helpful with this method. I do use this method when the tape needs to go on vertical or overhead surfaces. The tackiness helps hold the cloth in place as it's wetted out, which is a way to handle overhead or vertical work too.
Coating Raw Wood
All raw wood needs to be fully coat to be protected (encapsulated). This prevents rot and stabilizes moisture content, so the wood doesn't move or swell from moisture gain.
I use a technique that prevents "out gassing" (when air escapes from under the goo, leaving bubbles). I pour straight epoxy on the surface and move it around with a squeegee, putty knife or plastic applicator. As I move it around, I press firmly against the putty knife (or whatever) trying to mash the goo into the surface. This forces the epoxy into the wood pores. After I've covered a large area, I scrape the excess epoxy off the surface, usually moving it to other, yet to be coated areas, continuing to spread it out. The idea is to not have any areas, where epoxy has pooled on the surface. It's within these pools of goo that epoxy bubbles will form, so if your technique eliminates them, you don't have this problem.
Once I have everything completely covered and scraped of excess, I look for shiny pools, just to make sure I've gotten them all. This saves a lot of work later. This is only necessary with the first coat on raw wood.
If the just coated raw wood will receive a cloth sheathing, fillet or other epoxy/bonding or paint work, you're ready at this point to apply more goo. If the freshly coated raw wood will be finished bright (varnish), then you should wait until it's fully cured (24 hours), before continuing. Clear finishes will need a light sanding and possibly a different type of epoxy (extra clear), so you get a nice finish. This isn't the case under a fillet, cloth or paint.
The second coat of epoxy (yeah, you need a 2nd coat) doesn't need to be smashed into the surface, nor do you need to scrape it. With this coat, the goal is a uniform and even thickness coating. The first coat sealed the surface, this coat will add film thickness to help waterproof it. If the surface will be varnished, you might want to consider the special clear epoxies, like West System 207 (or similar). I use a squeegee to lay down these coatings, as it applies a nice even coat, that's easy to control. A roller does too, but it makes a bunch of bubbles, which you can tip off, but it seems you'll still be chasing them down with a blowtorch (pops the bubbles) and you never seem to get them all. A roller will waste a good bit of goo, but it is reliable at putting down an even coating. Brushes are the worst thing for applying epoxy evenly. They just don't leave a uniform coating, but in some places, like inside corners, you don't have much choice as it's all that will work.
With the epoxy applied you can tip off with a foam brush, to help level it out, but this isn't always necessary, especially on flat, horizontal surfaces, where epoxy's self leveling properties will do.
If you're going to apply a fillet or cloth, the second coat doesn't have to be as neat as a clear coat application. I just butter it onto the surface with a putty knife, spread it out, then drop the dry tape or cloth into the wet goo. The same applies to a fillet.
The goal for the pre-coats is to seal the wood and offer sufficient "film thickness". All epoxy formulators recommend a minimum of a 10 mil (1/4 mm) thickness, for insuring the coating is waterproof. This is the minimum, so the more the better (generally). Three solid coats will get this - the first is the sealing coat, with the next two building film thickness. If applying cloth, use a sealing coat and a single top coat, the wet out coat used to apply the cloth, will serve as the third.
Often you'll have to apply epoxy in odd places, such as inside holes, inside corners, under deck beams, etc. A brush can be helpful, but I've noticed a sponge works better. Yep, go to the kitchen and steal one of the old sponges, from the other half's stash (don't get caught, trust me on this). It doesn't have to be a pretty sponge, but an intact one that's not falling apart would be nice. Mix some goo and wetout the sponge. You can wipe under and around stuff really easily this way and though some epoxy will remain in the sponge, it's beats the hell out of a chip brush. You'll probably have goo dripping off your elbows if you've done much sponge work, so protect yourself appropriately.
When not to epoxy
Sometimes is better to not employ epoxy. Under varnish it can greatly add to the effort and upkeep of a brightly finished surface, eventually making sanding it down to raw wood a necessary. The same surface with just varnish or polyurethane finish wouldn't need this treatment or care, just because there's no epoxy to contend with.
A thin few coats of epoxy on raw wood will not appreciably improve it's stiffness, nor its strength. So, if this is your logic for using it, don't bother if you're going to clear coat over it. Now this said, on some softwoods a few coats of epoxy, will toughen the surface more than any clear coat (varnish or polyurethane), so it might be worth it. Sounds like gobbledygook doesn't it, but think about it. If you have a light spruce something or other, then drop a winch handle on it, will the addition of a few layers of epoxy help prevent a dent? I've found on softwoods, this can be the case, but on hardwoods, not so much, as these are sufficiently dense to resist most modest incidental impacts.
I also don't like to epoxy the outside of spars. Wooden spars take a beating, so they'll need to be refinished regularly and epoxy under the clear finish coats, just complicates this process. In some areas, like where a gaff jaw rests against the mast (the hounds area), I might wrap some light cloth and epoxy, just this area. This is for abrasion protection from the offending gaff jaw. If building a spar I will use epoxy on the inside, but the outside will be finished in a traditional way, if varnished. With the inside epoxied, moisture will be resisted, but the outside is still easy to maintain.
Laying Cloth in Goo
Sheathing is a necessary evil with plywood. It helps waterproof the wood and offers a good bit of abrasion resistance. A thin layer of cloth doesn't make the wood any stiffer, nor add any noticeable amount of strength. You need a fair bit of cloth to improve strength and especially stiffness.
In most cases cloth is just used to make it waterproof and to make the surface tougher. In this vein, there are fabrics that are a lot better than 'glass cloth, such as Dynel and Xynole. Both of these fabrics are a little harder to apply (they tend to float), but are significantly better at abrasion resistance. They cost about the same as regular 8 ounce (270 GSM) cloth and Xynole is the hands down better product, being 6 times tougher than 8 ounce cloth. Both of these non-'glass products will absorb a lot more resin than 'glass cloth, but they can be worth it.
To apply cloth there's a the wet and a dry method. I don't recommend the wet method, as I don't think the backyard build has a lot of places where it's very handy. The dry method has you apply the cloth over a previously sealed and coated surface (sanded if fully cured). Fit it dry and cut to shape as required. You may have to cut slits or pie wedges around curves and stuff, but it's stretchy, so don't get too crazy about this. I like to rough cut dry and fine tune the shapes/cuts when wet.
|Layout the fabric and remove wrinkles.|
Once the pieces are cut and ready to apply, position them on the part or hull, then pour straight epoxy in the middle of the cloth or area. Immediately move it around with a light touch using a squeegee or plastic applicator. You'll notice as you get the cloth fully wet, it goes transparent.
As soon as the goo fully wets the fabric, it'll go transparent.
This is what it's supposed to do, so chase down white spots and get them fully wet with goo. You'll move the cloth around a little bit doing this so watch what you're doing and stretch it back into position, if it gets too far out of whack.
Once the piece of cloth is fully wetted out, it'll look transparent, but may have pools of epoxy in places. Move these pools of goo to other places that might need more stickum. When you've finished wetting out an area, it should have a uniform texture. No pools of goo, no wrinkles, puckers, bubbles, white patches, etc. If you have these, work them flat. On curves, you'll have to slit and/or cut the cloth sometimes. I use a pair of scissors or brand new razor blade for this. Cut the fabric, then push it down, into the wet goo and flatten it out.
|When the epoxy is past the tacky stage, trim the excess fabric.|
With the cloth down you may need to trim areas where it's hanging over edges or something. Wait until the epoxy is no longer tacky and use a razor to slice off places you need to trim. Try not to lift the cloth from the surface in the process.
Once the now covered in cloth area is dry, you might have more trimming to do. I use an angle grinder with a diamond blade, like that used on ceramic tile. This cuts cured goo and cloth like a charm. Once trimmed up, it's time to fill the weave. Some will tell you to just apply more straight epoxy, but this just wastes resin. The easy way is to fill the weave with a cosmetic fillet mixture of microballoons, Q-cells, quartz spheres, etc., plus some silica to control viscosity (important on vertical and overhead surfaces). Smear this mixture all over the cloth with a putty knife or squeegee, in a thin, uniform layer. You'll sand much of this off in the fairing process, you're just looking to fill up the weave.Of course if you're looking for a natural finish, you have to fill the weave with straight resin.
|Resin filled weave for a natural finish.|
Fairing and Smoothing
These operations are the butt kicker in boat building. You will cuss (a lot) and your elbows will be pissed at you, but the more effort here, the better the boat will look.
Smoothing and fairing aren't the same. Smooth is what you feel with your hand, like that on a baby's butt - fair is what you see, like a dent in a car door. A car door with a dent can be smooth, but it's not fair, while a fair surface doesn't necessarily mean it's smooth.
Fairing is done first and the easy way is to lightly coat the area with a light colored primer (yep paint). It doesn't have to be a real pretty coating, just enough to offer some color on the surface. Most of us just use an aerosol and lightly "mist" the area, but a rolled or brushed or even sprayed primer works too. The next step is the "long board" (the board of pain and an image is shown in the Painting section). Starting at one end, sand at an angle, say about 30 degrees and sand only at this angle for each stroke, the full length of the area. This places a diagonal scratch in the primer. You're not looking to remove much material, just scratch the primer, so you can see what's going on. After the first pass, go back down the same area at the opposite angle. Now you'll have a crosshatch pattern across the whole surface. The long board will bridge over the hollow and dented areas, while knocking the tops off the high spots. This instantly tells you where you need to focus your efforts. Some places will need to be filled (low spots), while other places will need to be knocked down a tad (high spots). Mark these areas and as a general rule, you'll bring the lows up to meet the whittled down high spots. Apply a cosmetic filler to the lows, rough them down flat(ish), then do the primer cross hatch thing again. This time working the areas you know are too high or where you just filled a little, to bring them in line.
I change primer colors for each pass, so I can keep track of what I've added or removed. It also helps greatly if your lighting is at a low angle, so you can see the shadows of the low spots. It's actually possible to have too much light, which "washes out" these imperfections and you can't see them very well.
|An ideal light arrangement offers a low angle, so you can see the low spots |
and the quality of the fairing/smoothing. Also note it's not very bright in
this booth, so the light doesn't flood out the imperfections.
I like the light behind and to each side of me during fairing, smoothing and painting situations. This permits the shadows of low spots (depressions) or bumps to be easily seen.
A pro will make three passes with the long board and call it done, but the average guy can make a career trying to get it fair. It helps to stop, primer the whole area, then wet it out (with water) so you can better see imperfections. With practice, your hands will tell you a lot more than your eyes, but this takes a boat or two.
Smoothing is a surface treatment, once you have things faired up. I do most smoothing operations once the paint is building up, including top coats, unless it's a fancy paint job, where I'll smooth the primer first. This is because primer is cheaper and easier to smooth.
A typical top coat smoothing job will have you progress from 220 grit, through 400. I prefer to wet sand these, as you can be neater, it keeps down dust and you can see things better. Once at 400 grit, the surface is pretty damn smooth, but if you're a nut job or just real anal, bust into the finer grits up to about 2,000. This is baby's butt smooth, though some will want it better still, so you'll need a cutting compound and a buffer. I use a two stage cutting compound, one rough and the last fine. Next I dig out the polish and hit it again with the buffer. I'll work the paint until I can clearly see myself in the reflection. The only problem with these types of finishes is, it'll show each and every slight imperfection you missed during the fairing process, so be careful what you wish for.
All epoxies blush (wax like film left on the cured surface), even the ones that aren't supposed to. Amine blush (that's what it's called) is a water soluble, environmental reaction with moisture in the air and is easily removed. The only safe and sure fire way to insure this isn't a problem is to wash any and all surfaces that have cured epoxy on them. Don't use solvents, as you'll just smear it around (water with a touch of dish detergent soap works good). I like to kill two birds with the same stone, so as I'm washing the surface, I use a Scotch Brite pad, while I'm scrubbing. This scuffs up the surface for the next coat as it cleans off the blush. You don't have to do this. You could just wash and dry the area and sand it later as required. My basic rule is to assume there's some blush, even if just a little (really screws with paint BTW), so I wash everything (yeah, I'm nuts).
This is just a quick overview of the methods, techniques and tricks we use. You'd be best advised to download the free "User's Guides" from www.WestSystem.com and the "Epoxy Book" from www.SystemThree.com. These will cover things in more detail, plus have more techniques and methods. Also West System has a free book to download called "The Gougeon Brothers on BoatBuilding", which is a great reference.