Liquid Joinery

I think Dave Gerr coined this phrase and it's an appropriate one for sure. It's literally joints, be they structural or cosmetic, bonded together without fasteners, fancy and tight fitting woodworking, etc. You can just about use a hatchet to cut the wood elements apart, slather on some goo, stick them together and bingo, a solid joint that can be ground down and painted, so no one but you would know what a wood butcher you are. 
   In fact, it's best if the joints aren't all that well fitted (yep, some slop is good). All that's really necessary are the pieces aligned the way you want, once everything is all cured. Given this, when goo hits something, it will conspire against your best efforts to keep them aligned the way you want. This is why many think it's evil. A few well placed temporary brads, nails, screws or even a concrete block to keep the parts where they need to be, for the cure is a real good idea. Don't let the epoxy beat you up, you are the master, so show it who's boss.
   The partially completed joint below is a good example of this sloppy joint approuch. You see the edges are back cut a little, which exposes the end grain to the epoxy, helping seal it up and of course, the top and bottom "faces" are "hollowed out" enough to permit the fabric and epoxy to live flush, when finished. This page is intended to offer some guidelines to making these joints, be they a chine seam, inside or outside corner or simply a flat seam that needs to be joined. The dimensions and radius's are for the average trailer sailor and a good guide. On lighter  and smaller boats, where weight is a concern, the number and amount of fabrics employed can be reduced a bit, as well as the overlaps, but in most plans this is typically covered in the scantlings or construction sections (drawings).

Click on the drawings to enlarge them.

 The Payson Butt Joint
- As shown with only the top completed, this joint replaces scarfs and butt blocks.
- One layer of 6 oz. biax per side for 1/4” plywood.
- Two layers of 6 oz. biax per side (or one layer of 12 oz, per side) for 3/8” plywood.
- Two layers of 12 oz. biax per side for ½”plywood.
- Biax tape used in structural applications, or if plywood will bend considerably.
- Regular ‘glass cloth can be substituted in lightly loaded areas.
- Biax should be 45/45 style weave.
- 6:1 slope ratio is fine for most joints. Increase on highly loaded joints or areas that receive considerable bend.
- Tapers and back cuts do not have to be precise, a rough rasp finish is perfect.

Structural Fillet Anatomy

Mixture ratios can be all over the place, depending on humidity, temperature, what you're doing, etc. Generally you'll use silica to make the mix the thickness you want, after you've mixed in the other stuff, which often are "bulking agents" or fibrous reinforcements. Though I suggest wood flour here, I've stopped using this in heavy structural bonds and fillets. I prefer milled fibers, cotton flock and other materials that don't let the goo drain out, when the exothermic reaction kicks in, like some wood flour can. I do use wood flour to match colors, which is handy with brightly finished jobs.
- Fillet mix, ~40/60 silica and milled fibers or wood flour (add silica to stiffen mixture).
- Fillet extends 3 times plywood thickness in each direction, from centerline of joint.
- Inside corner receives two layers of 12 ounce biax, inner layer overlaps 3 times plywood thickness, outer layer overlaps 6 times (see drawing).
- Outer corner, radiused 3 times plywood thickness.
- Outer corner receives 1 layer of 10 ounce biax with 6 times the plywood thickness overlap (plus sheathing if desired)

Fillet Types

There are three basic fillet types; heavy structural, light structural and cosmetic.
   All fillets have a thickened epoxy mixture on the joint. The actual mix is dependent on the fillet type. Heavy structural fillets are 40/60 silica and milled ‘glass fibers, light structural fillets have the same mixture with some wood flour or micro spheres (Q cells, micro balloons, etc.) added to ease sanding effort. Cosmetic fillets are mostly light materials, such as Q cells, for easy sanding, though silica may be added to control viscosity. The amounts and ratios are something you'll have to work out, but it doesn't take long to get an idea how much of what you'll need for the job.
   A heavy structural fillet has 2 layers of biax on the inside and at least one layer of biax on the exterior of the joint. These types of fillets are usually along the hull planking seams and other highly loaded areas, like centerboard cases, bulkheads and keels.
   A light structural fillet is much like the heavy, except a single layer of biax is used on both sides of the joint. Regular cloth of similar weight, can be substituted for biax in most light structural fillets, but only on the outside of the joint in most situations.
   Cosmetic fillets are decorative in nature and don’t require fabric over them, though many will for abrasion resistance. Regular cloth is used, not biax.
   Filler materials for a cosmetic fillet are selected for ease of sanding, sometimes color (natural finishes) and workability. Q cells, microballoons, wood flour, talc (yep, baby powder) etc., are all common materials. These types of fillets are just to round off inside corners to make cleaning easier, to shed water, hide seams, prevent trapped moisture, etc.  If the fillet is to provide strength to anything, it will need silica, milled fibers or other “fibrous” material added to improve strength.
   As shown above, it’s often wise to “back cut” the pieces to be filleted. This allows the end grain to be covered in epoxy, protecting it. This doesn’t have to be neat, I use a big grinder or belt sander to whack off the edges on these back cuts. So long  as the panel alignment is good, the edges (gaps) can be off by a 1/2”. Epoxy fills these gaps easily and makes the joint stronger too.
   Tape overlap and height of the fillet are minimums. Other than extra epoxy, there is no problem with additional materials.
   As a rule, you don't need to be particularly precise about these joints. The keys to remember are to back cut the contact area, so the goo can get in and around the end grain and of course, the appropriate heights for the fillets and fabric schedule (if required) applied over them. The inside corner of a joint is the most important, in terms of strength and stiffness, but this doesn't mean forget about the outside. What I'm saying is, you can use the hull sheathing, to supply the fabric overlay on a chine for example, because this is an outside corner. On heavy structural fillets, the inside will always need fabric, with plenty of staggered overlap. Back to neatness of the raw wood elements, you can hack it out with an ax, so long as alignment is where you want. The epoxy will fill, seal and bond the most hacked of joints, which under paint is invisible. Naturally, if you are looking to create a varnish finish, this isn't a good looking joint, so a mechanical scarf  or butt block would be more desirable.
   Exceptions to these general guidelines to fillet heights and tape schedules are on lightly loaded joints and/or little boats. A canoe or kayak doesn't need the 3:1 rule and in designs where every ounce counts, you can reduce fillet heights and tape weights/amounts. How much to reduce, well this is the butt kicker, but 1.5:1 (half the recommendation isn't unusual and I've seen less. If treading this fine line, perform some tests and see if your reduced dimensions will still have a failure within the substrate, not along the glue line.