Hi I’m pretty new to woodworking. This is probably a pretty basic question. I’m making a desk with just cheap pine that I cleaned up. I’ll be using castle joints to join the legs & frame. Should I stain all the pieces before final assembly & gluing, or should I glue it the stain it? Same for the tabletop, stain the entire top & bottom, or will the stain inhibit the glue from working? Thanks a lot.
Staining the wood, or finishing of any kind, will weaken any glue joints as the glue won’t be able to seep into the wood as effectively.
The best glue joint is bare wood to bare wood, and long grain to long grain.
That said, if you have parts that are difficult to get to once assembled you could part stain first, but if you do, mask off the joint areas so no stain gets on them (same point as above). I would only do this as a last resort.
In several of his videos Mr. Tom Fidgen applies various stains and finishes (such as iron oxide, shellac, and bog oak) prior to gluing up. Other woodworkers go to great lenghts in protecting the joint surfaces when applying shellac before gluing-up.
Hide glue and PVA glue undergo cohesion by dissipation of water, which probably would hindered by an occlusive finish like polyurethane varnish and paint, but perhaps not by stains or shellac, as they are more forgiving in allowing water to penetrate.
As for the adhesion of the glue to the pieces that are to be bound, there seems to be relatively small amount of evidence.
The chemistry of PVA glues are a bit more complex than just dissipating water. The poly Vinyl Acetate is an emulsion in water, which greatly slows down its ability to polymerize, or form long acetate strands. When water leaves, the chemistry of the glue is different, and not readily reversible. And for this reason PVA doesn’t adhere well to dry PVA, and it’s fairly resistant to water, oil, or greases.
Hide and animal glues are mostly reversible, but PVA and it’s ilk can’t be readily reversed.
It’s also why PVA has a shelf life. Even when mixed with water in the sealed tube, the glue will eventually form long stringy molecular chains.