Plywood is used for making many different things, from furniture to homes. In many cases, the plywood itself is not left exposed, but used as a substrate, providing structural strength and a flat surface to adhere the finish materials to. But how well those materials adhere to the plywood may not be as good as we like.
There are several things that can get in the way of adhesives or mastics sticking to the surface of the plywood. But basically, they break down into a few categories:
• Texture – Most plywood, other than luan and cabinet grade plywood does not provide a totally smooth surface. This causes problems with some things, like vinyl lettering, which need full surface contact with the substrate to adhere well. Solving this problem requires filling the surface of the plywood and sanding it smooth.
• Moisture – Plywood direct from the mill often contains too high a moisture content to guarantee good adhesion. This is easily solved by allowing the plywood to dry to a moisture content of 12 percent, before applying the finish material.
• Porosity – Wood in general provides a porous surface. Since most plywood is made of soft woods, the surface is even more porous than hardwood plywood would be. This causes the wetting agents of the adhesive to soak into the surface of the wood, reducing the effectiveness of the adhesive.
• Chemicals – In the case of pressure treated plywood, the chemicals that the plywood is treated with cause adhesion problems. Washing the plywood with soap and water and then allowing it to dry for a couple of weeks, before painting, will eliminate this problem.
• Flexibility – If too thin a piece of plywood is used as a substrate, it may flex from the weight of people walking on it, causing cracking in adhesive. This problem is especially prevalent when installing ceramic tile. It is always necessary to ensure that a sufficiently thick piece of plywood is used to provide rigidity.
In most cases, plywood should be sealed with a quality primer-sealer before applying any other finish material to the plywood. The primer-sealer eliminates the problem of porosity and the chemicals from pressure treated wood, providing a good surface for adhesives to stick to. The one great exception to this would be when finishing hardwood plywood in furniture and cabinets.
When installing ceramic tile over plywood, always use a pre-mixed flexible based adhesive for the tile, not the type that you mix in a tub on site. These are rigid adhesives, which will not survive the normal flexion of the plywood.
Vinyl tiles offer a different problem than ceramic tiles. They are naturally flexible, so they don’t have any problem with cracking when the plywood flexes. The problem with them is gaining good adhesion. If the substrate is an exterior grade plywood, then a thin layer of finish plywood should be applied, before the vinyl tile. One-quarter inch of “A” finished plywood (not luan) will provide an excellent substrate.
Prime this thin layer of plywood before installing the tile, allowing the primer to dry fully. Vinyl tile should always be rolled with a 75 or 100 pound roller after installing, to ensure full contact between the adhesive and the substrate.
Even tapes have a problem sticking effectively to plywood, due to the same problems of texture and moisture. There are some tapes, specifically, those designed for rough surfaces, which work well with plywood. Gorilla brand and 3M both produce duct tapes that are specifically designed for use with rough surfaces.