Filling knot hole with cyanoacrylate glue

What is the best way to fill a hole under veneer?

I am in the process of building a coffee table, and I made the lower shelf using oak veneer plywood. After assembling, I found there is a spot about the size of a quarter (there must have been a knothole in the lower level) where the veneer is spongy and has cracked. My question is: what is the best way to fill under the veneer? My thought was to take a syringe and inject the hole with glue. But I wasn’t sure if, when it dried, it would shrink and cause a low spot. Any suggestions? - Curtis McCause

Rob Johnstone: You could inject hot hide glue with a glue syringe, as hide glue would not shrink when it cures. The same is true of cyanoacrylate (CA) glue. White or yellow glues will indeed shrink as you feared. One detail to consider: if you are going to be staining the piece, I would do that before you tried to fix the soft spot. Any glue you put in there will seal the fibers of the wood grain and keep the stain from being absorbed evenly. Once the stain is on, you can give the fix a try. I can't guarantee that it will work, but there is a every likelihood that it will.

Tim Inman: This is a technically challenging repair. From your question, it appears you have not yet finished over the flaw. In that case, I think you could infuse something like hide glue or CA into the cavity and support it from beneath with reasonable success. CA would be my first choice, but don't make it your first choice unless you have previous good experience with it. I use the alcohol thin versions of CA for this. After it hardens, use a piece of sandpaper on a FLAT board to cut off any excess and to level the surface. A nice, thick, hot charge of animal hide glue would be the most forgiving and easiest to use material. That might be my best recommendation for you. Of course, you could always use a matched patch cutter on a router and cut out the damaged area and fill back in with a carefully matched patch veneer piece.

In my own restoration work, I use a vacuum system to do this kind of repair. I have developed a technique to allow me to bring the benefits of vacuum to very small areas on furniture -- without the need to encase the entire piece in a vac bag. Using vacuum, I am able to bring fillers like epoxy or polyester into the void and completely fill it from beneath, with a perfectly level surface on top. It is a superior way to do these repairs. I mention it here just to let you and others know that there are advanced techniques that are quite successful. They do require experience, practice and a considerable investment in equipment. Maybe this is not something for a "one-off" repair in a hobby shop, but it is a method to know about "just in case" for future reference.