While plywood is an amazingly strong engineered material, it is not impervious to damage. It would be nice if it was, but that’s just not in the cards. Rather, we need to be ready to repair plywood that has been damaged, especially when it has been damaged in a critical area.
There are a number of different types of damage that plywood can incur, but I’m specifically referring to dents, surface cracks and bubbles. Water damage, which attacks the edges of the plywood first, generally requires replacement of the piece (although reinforcement is possible in some cases). Cracks that go all the way through the plywood are virtually impossible, due to the way plywood is made. The next layer of veneer would check the crack, regardless of how seriously the surface was cracked. Major damage, where a piece of plywood is broken pretty much always requires replacement as well.
Dealing with Bubbles
Bubbles form in plywood when there is a localized failure of the adhesive used to attach the layers of veneer together. Moisture enters into the wood through an unfinished side or through pinholes in the finish and dampens the surface veneer, causing it to expand. As the wood seeks space in which to expand, the bubble is formed.
Since the bubble is only in the surface veneer, it does not destroy the structural integrity of the plywood or what has been made out of it. However, it can expand or the bubbled veneer layer can begin to break if not taken care of. In boats and other structures that are subject to water, repairs should be accomplished as soon as possible.
The delaminated veneer of the bubble needs to be removed from the plywood in order to effect repairs. This is best done by cutting around the edges with a utility knife or chisel, cutting out an area slightly larger than the bubble itself. Be careful as you cut, so as to not damage the veneer layer beneath. Allow the area to dry, before filling the dent left by removing the outer layer of veneer.
Repairing Dents and Cracks
Dents, surface cracks and the indentations left by bubbles can be filled with high-solids epoxy or epoxy putty. You should always slightly overfill the indentation, as the epoxy will shrink slightly while setting. Once set, the epoxy can be sanded, just as if it were wood. Please note though, that it cuts a bit slower than wood, so if you are not careful, you will take more material off from the plywood around the repair than in the repair itself. As long as the sander is sitting flat on the surface, this should not be a problem.
Some people are satisfied with the finish that they are able to achieve by simply filling the indentation with epoxy and sanding it. However, on larger indentations, such as those from bubbles, a layer of fiberglass cloth should be put over the repair to help it blend with the surrounding surface. This layer of cloth can either be attached with epoxy or with fiberglass resin. Epoxy is more flexible, so it will resist delamination caused by flexion better.
Paint the epoxy onto the repair area, place the fiberglass cloth on it, sticking it down and then cover it with another layer of epoxy. Once cured, the epoxy and fiberglass cloth should be sanded, feathering it out to blend with the adjacent surface. Finish the repair to match the plywood’s original finish.