I need a 12’ x 4’ length of plywood for the walls of a teardrop trailer that I am building. I am going to be joining two smaller pieces together to get the desired length.
I’ve decided to go with a butt joint that is reinforced on the inside by a backing piece of lumber (see figure 4 in this article: http://www.boatbuilding-links.de/Jim-Michalak/joining-plywood-sheets.html). It seems like the easiest approach and the one most likely to meet with success taking into account the tools I own and my experience level.
Two questions:
(1) What would be good dimensions for the lumber butt plate that holds the two pieces together? What thickness and how much of an overlap is needed for good strength?
(2) What would be the best glue to use for this sort of thing?
If it is a structural piece, the butt plate should be no less than 6" wide, and the same thickness as the plywood it’s holding together. You should also skarf (scarph?) the joint, i.e. not cut on a 90 degree angle to the face, but at at least 45. See figure 1. Or use buscuits as well as the butt plate.
As for glue, Probond interior/exterior wouldbe fine, just make sure you spread the glue over the whole joining surface, dont just run a bead of glue and stick the pieces together.
A 6 inch strip seems excessive to me, even for an unscarfed butt joint, but if it’s not to be seen, there’s no reason to under-engineer it, so I would proceed as follows:
Lay a sheet of thick polythene on a very flat floor, or on a sheet of ply - the sheet must be longer than your joint.
Lay the two sheets of ply together so that the butt joint is on top of the polythene - push a couple of matchsticks in between to create a gap.
mix up some slow-setting pre-thickened epoxy, scrape it into the gap between the two boards using a strip of plastic cut from a margarine container or similar - make sure the gap is filled
Coat one side of your 6 inch strip with a thin layer of epoxy
remove the matchsticks from between the two boards and press them together until they are nearly joined - spread out the glue that is exuded
Apply the butt strip to the joint, double-check the alignment and register of the underlying boards, then apply lots of weights to the unglued inner side of the butt strip - paint pots, bricks, cans of food, whatever you can find.
Allow to cure. The epoxy will not stick to the polythene sheeting - so as long as that was flat against a smooth floor or something, it should leave a very neat, smooth joint when it just peels off - it may not need any sanding at all on the outside face of the joint.