Any glue that you use will only stick to the shellac. The shellac to wood
bond is the most strength the joint will have. In that case, glue it with
more shellac. It will be weak but no weaker than any other glue you use.
To get a good bond, the shellac will have to be removed or an alternative
method used. Shellac is used by some woodworkers to put on surfaces where
they don't want the glue to stick (through tenons for example), so I'm
pretty sure you're going to have problems if you try to apply glue over the
As an alternative, could you cut biscuit slots and use biscuits? If you're
gluing face-to-face, face-frame ("FF") biscuits should work okay. If you're
edge-gluing the plywood, then biscuits are probably called for already and
would be ideal. If biscuits won't work, then I'm sorry, but I don't have a
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