By the way, this is on the deck of the Kaholo paddleboard.

The epoxy shouldn't add any thickness to the joint. Keep the mixture pretty thin - like cheap ketchup - and don't lay it on very thick. Make sure the pieces are properly aligned - this is the most critical thing. Then clamp the joint or press it under some weight. It doesn't need to be perfect, as a scarf joint is far stronger than necessary for our purposes. Even if there are some voids in the joint, it will be totally encased in fiberglass anyway. -Wes

The epoxy won't raise the surface at all.  Think of the scarf joint as two opposing wedges.  The epoxy will be a thin layer between the mating surfaces, displacing the planks a tiny bit longitundinally.  It won't displace them vertically unless you overlap the pieces too much.  Done right, the top and bottom surfaces of the planks will be exactly even, and their combined length will be a tiny, tiny (as in, insignificant) bit longer than a dry fit would suggest.

Don't worry about planning this out; the epoxy is a pretty good lubricant before it cures, and it will "want" to make the parts slip apart the right amount.

The key point is the one Wes raises above: make sure the pieces are properly aligned, and clamped securely. 

By the way, don't clamp too tighly, or you'll force the epoxy out and the joint will be the weaker for it.

I've read (here and elsewhere, IIRC) about people doing butt joints with one or two layers of glass on the interior hull side (i.e., a 6" wide piece, followed by a 3" wide strip)... and most of what I've read on this approach has been positive. 

Any suggestions?  My rationale is that some (here and elsewhere) have complained about the appearance of scarf joints if not done precisely.

Thanks,

Larry

Yup, butt joints are a good and valid way to go when joining long hull & deck panels. Glue the butt joint together with thickened resin, put a strip of glass cloth (about 3-4" wide) on one side, clamp it down under a smooth plastic sheet, when cured, turn it over and do the same on the other side.  The glass strips are then feathered out with sandpaper and are invisible under the final glassing of the hull.  Good idea to use wood flour to glue the butt joint so that the fine line is the same color as the surrounding wood.  If you use silica, the line may be yellow and contrast rather than blend in.  That applies to scarf and puzzle joints, too. See the process in my gallery under Arctic Tern Project.  http://gallery.me.com/jermcmanus  Build-on.  Jer

Just because two methods work in a given situation does not mean they are ,necessarily, equal.

I built a  S&G pirogue with plywood that only came in half- sheets. Each panel of the hull had three scarfs running thru' it. Without any interior structure the hull was so stiff and sturdey that the scarfs all survived when the boat flew off my roof-rack and landed in the road. Try that with your glass covered butt joints.

Scarfing is the woodworkers friend. It's  potential applications are almost limitless. Once you learn to do it you are set for life. Wanna' build an 18' SOF kayak out of 8' two-by-fours? If you can scarf it's  a no-brainer.

Scarfing takes a little technique which you can acquire by doing some test pieces using scraps before you do something you will have to look at the rest of your life.