This is an extensive subject, ranging across different substrates, paint types and formulations and application techniques, with brush, roller, spray or a combination (roll and tip, etc.). I’ll try to cover the basics, most backyard builders will want to attempt, so forget about $2,000 worth of compressor, regulator, filters and other related spray equipment. I have a fancy 7 HP 15 CFM (at 90 PSI) compressor and a 30 year old “Binks” that I love and have rebuilt a few times. It’s the classic siphon feed cup gun and does automotive type of spraying. I also have gravity fed HVLP guns that can spray anything from melted tar to the thinnest paint. You don't need any of this stuff, but let’s start with the major elements of painting. 
    The art of painting anything is simply prep. All good paint jobs are 90%+ prep and the actual hand on brush/roller or sprayer time is what’s left. I just “shot” a little boat, 15’ (4.57 m) long, maybe 4’ (1.21 m) wide. The two top coats took about 20 minutes each; the 3 primer coats about the same, so a total of just over an hour and a half to apply all the paint. I had about 8 hours in smoothing the primer, another 8 hours in fairing the surface, before the primer went down and probably another hour just fixing screw holes and other divots in the surface, after the hull was sealed with epoxy. This doesn't count the epoxy coating time portion of the paint job. So, you can see, you'll put a lot more time in the surface prep than paint application. 
    Epoxy coating is a different subject, but has some relevance here. Some alkyds (oil based) can react poorly over an epoxy coated surface. It depends on the formulations of both the paint and epoxy. Worse yet is, you can't tell which will have a reaction and which will not, without a chemical engineering degree and close examination of the various product contents. So, if planning on an alkyd top coat over epoxy, use an epoxy primer. In fact, you'll hear this epoxy primer recommendation over and over, simply because it’s going to work, regardless of what you put over it. You can take a chance with some other type of primer, but it’s a chance and you might wish like hell you'd bellied up for the epoxy primer, particularly when large cured sheets of paint are peeling off your pride and joy, because you cheaped out on the primer. The primer is one of the keys to a good prep job, so this isn't the place to save money. 
    There are two types of primers: a surface sealer and a blocking type. The surface sealer is pretty thin and not really designed to get sanded much, just promote adhesion to the substrate. A blocking type has silica (and other stuff) in it, which bulks up the paint (it’s usually thicker), so you can sand it down to smooth the surface afterward. Unless you're the world’s greatest substrate prep person, you'll want the blocking primer (also called sanding, bulking, blocking, building, etc.). These thicker primers will fill pretty big scratches in the substrate, as low as 100 grit, with 2 unmolested coats. I prefer to take it to a minimum of 120 grit and rarely go as high as 150 grit. Primer, especially the thicker blocking types need lots of “tooth” to grab a hold of. These primers aren't like automotive primers were using 400 grit is normal. At 150 grit it’s smooth enough and still have the paint stick good. Much over this and you run the risk of insufficient tooth. System Three sells water “borne” epoxy primer that many like. Like all epoxy primers, make sure you get your sanding done within a 3 day window after it’s dried. If you don’t, the primer gets really hard to sand, dramatically more than if sanded within the first few days after it’s cured. 
    I apply lots of primer, in thin coats and very often 4 or more. This is partly because it’s thin, but mostly because I'll be sanding a few layers off, as I smooth things up, so I need the extra “film thickness”. This film thickness thing is important, as it’s the physical amount of protection your surface has. A good amount to shoot for is 10 mils (.25 mm) as the final thickness of primer. A single brushed coat of sandable primer goes down about 8 mils (.2 mm) thick, but dries to about half this, so two coats will get you in the ballpark. If you need to sand and smooth the surface to fill minor imperfections, you'll need at least one more coat, because you’ll sand through a layer while sanding. This is why I apply several coats of primer.
   Usually my routine is to apply a few coats initially, sand to find low and high spots, fix the low and high spots with more spot applied primer, then wholesale prime the area again, for final smoothing. I'll also use different colors when priming, so I can see what I'm doing and which layers I've sanded down through. Ultimately, I'll want at least 10 mils of primer over everything.

A homemade longboard will greatly help with fairing and smoothing.
Make from plywood or acrylic sheet and wrap paper around to
the clamps on the end.
    Once you've gotten to the point of primer, you've pretty much made sure the surface is fair, but very slight imperfections may still exist. You use the sanding techniques as you did with fairing, to find low spots in smoothing. Using a modest grit maybe 180, lightly sand on an angle, along the whole area and in the same direction and angle. Next come back in the opposite direction, again sanding at the reciprocal angle. This forms a cross hatch pattern of light scratches. You're not trying to remove much material, just put a light scratch on the surface. You'll be able to see the low spots, as areas where there are no scratches. These areas need to be filled with more blocking primer and sanded down flush with surrounding areas. This is tedious, but having a low light angle helps a lot, so you can see the shadows of the low spots. Arrange a light beside you and close to the work, but not a really bright light, because you don't want to “wash out” the shadows. The low angle light is like a sunset, when the shadows are long and the intensity not so harsh. 
A professional booth, with side and overhead
lights that can be switched separately.
A DIY booth is easy and cheap. 
Note the furnace filter for clean air.
    Once you're satisfied the surface is smooth, apply one last coat and wet sand with a 220 grit paper wrapped around a soft pad. This last coat insures everything is sealed, you've replaced what you've sanded away and the wetness will let you see the surface imperfections better. You can make a career out of this sort of thing, but eventually, you'll just grow elbow weary and move on. With experience, you'll be able to get it done more quickly, but initially, you'll move a lot of primer around, just to find more areas that need attention. 
    Let the primer dry for at least 3 days. It'll shrink a bit, so you need to wait. Yeah, it sucks, but trust me it’s worth the trouble and time. I usually wait as long as possible, a week normally, to prevent shrinking from screwing with the top coats. 
    The boat should now be one color, fair and smooth, congratulations. Now get over it, you're not done yet, so get back to work. The same application principles apply with finish coats (top coats), you apply a couple/few coats, lightly sand, looking for imperfections or trying to remove the suicidal bugs, that appear just after you open the paint can. I almost always wet sand finish coats, simply because I can see things better in the shiny, wet paint. If you were overly anal smoothing the primer, your finish coats will look pretty darn good. If on the other hand, you're like most novice painters, you discover it’s not very smooth, once the shinny paint goes down, you fill the low spots and sand some more (just what your elbow wants to hear), just like the primer process. 
    Applying paint is usually brush, roll and tip or spray. The roll and tip method saves you the bother of a spray gun outfit, with nearly as good of results. Modern paints will self level to such a degree, even brushed surfaces look really good. 
     Most know how to use a brush. The only real rule is to “maintain a wet edge”. What this means is work in small enough areas, with the brush, so the previously painted area is still wet enough to accept new brush strokes from an adjacent area, without leaving a mark. This means pull the new paint into the just applied paint, going from dry to wet. This helps you align brush strokes and blends in the freshly painted area, to the just painted area. If you don’t you’ll see “lap marks” which still out like a sore thumb on painted surfaces. The same rule applies to the other methods, keep the wet edge alive. 
    Roll and tipping is the popular way now and it works with these newer self leveling paints. Roll out an area you can handle. Not a big area, especially if it’s hot in the shop, just big enough so you can roll it, tip it and most importantly, maintain the wet edge when you move to the next area. In most cases this means a 24” square (.6 m) , maybe smaller if it’s hot out. Roll up one edge of this square, then diagonally down to the other corner, then straight up again, forming an “N” shape. This leaves two triangular shaped spots with no paint. Place the roller in the freshly applied paint and push it into the two triangles. Now you’ve covered the 24” square, but it’s not even yet, so go over it a few times with even pressure to distribute the paint evenly across the whole square. This is the nice thing about a roller; it can apply paint in a uniform thickness. Put the roller down and pick up the brush, which should be dry. Hold it nearly vertical to the work and with a light touch, glide it over the fresh paint, with just enough pressure to remove the roller marks and stipple. Go from one side to the other, each stroke being the same pressure, direction and speed. If it loads up with paint wipe it off and keep going. Repeat these steps in an adjacent area, overlapping the roller and brush strokes a wee bit as you go. Congratulations, you've just rolled and tipped.

The Graco 3900 and typical of these DIY units.
    Now, the really best method is spraying. Well damn, you said I didn't need this stuff. Yep, but it's the best way and there's “homeowner's” grade stuff available now that’s cheap and produces a fine finish. Everyone's familiar with the “hand buzzer” style of airless sprayers. These suck and wear out your arm quickly. A new type has shown up and they work well, for less than a $100 bucks if you hit a sale. These are the HVLP (High Volume Low Pressure) turbine units. These have a vacuum cleaner looking thing with a long hose on it, attached to a handheld gun and a paint cup. Air, generated in the turbine unit, goes up the hose, pressurizes the paint cup, which is then forced out the gun's tip. The better ones have flow control and interchangeable tips and a fine finish is possible with these, given a little practice. I recommend the Graco 2900 HVLP or 3900 HVLP units. The Wagner's just don't hold up well and there are other brands, but cheaply made. Don't get me wrong, there are some others that work good too, I just haven't had much luck with those I've tried, but have enjoyed the Graco pieces. 
    These homeowner sprayers can apply paint in various spray patterns from about a 1” (25mm) to a several inches, which can be handy. You can control the finish, to have perfect coatings without sanding or buffing and it'll speed up the application process. Naturally, though they're not bad about over spray, you'll still get some, so mask the crap out of everything you don't want paint on. When spraying acrylics (latex) with these puppies, the warm air coming from the turbine, will dry the paint much faster too. 
   The homeowner grade versions of these "turbine" units will not lay down a finish as good as the production guns used in body shops, but they are much better than brush or roller. With some practice, you can get pretty good results with these inexpensive assemblies, but don't expect them to hold up to a lot of painting. This is where you need to step up and get a real production setup. Of these there are the turbines (typically multiple turbines) and the compressor air supply types. The good ones aren't cheap in either style, but the multi turbine units are coming down in price and rising up in finish quality to rival the compressor guns.
   In this regard the WR-LPU stuff from System Three has difficulty.
I've never had any luck spraying this type of paint. It just dries too fast and I've made a mess. You can still brush or roll and tip it, but I like the finishes I can get with spraying, plus the application speed. These turbine guns work fine on solvent based LPU’s and other paints, just not the WR-LPU’s, from my experence. 
    This leads us to paint. The choice of paint seems endless. In recent years huge advances in formulation have occurred, so all paints are better than they once were. Straight alkyds are now modified with acrylic urethanes or polyurethane and some are epoxy based. The acrylics have drying, curing and flow agents in them, so they “lay down” nice and level out to a smooth, brush stroke free (or nearly so) surface. Then there are the specialty paints, like the LPU’s and acrylic urethanes and polyurethanes. The LPU’s are the most expensive and most difficult to apply, with the acrylic urethanes just behind. These are two part paints, often with separate clear over coats (like automotive paint systems) you can apply over the “base” paint. AwlGrip is a common brand, as is Perfection from Interlux. These lay down sweet and have great gloss and durability. Considering their cost per gallon, they darn well better.

    The next step down from the fancy, expensive stuff is the single part polyurethanes. BrightSides is a common brand, again from Interlux. These are much less costly and nearly as good as the two part stock. They also are easier to apply. This is my usual recommendation for the DIY'er, especially if you'll brush or roll and tip it. 
    Jumping further down the list are the house paints, both acrylic (most common) and the alkyds (old school oils). These represent the budget minded builder and you can still get a good finish if the prep is well done. Porch and Deck enamel is the most durable among these choices, either acrylic or alkyd. The Acrylics are less prone to mold and mildew, so use this inside lockers and under V berths. If looking for a real marine paint, Rustoleum has a pretty good product. It's a traditional oil base and easy to work with. 
    Follow the instructions on the can, for thinners, wetting agents, flow promoters, etc. Although you can make substitutions, you do need to understand the formulation chemistry, to do a good job of this.

    I have no affiliation with any of the products I've listed, just offering my opinion and experience with them.

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