Plywood is an inexpensive wood alternative – especially if you need pieces of a larger width. And if you are putting your plywood outdoors, it is also really easy to waterproof!
Plywood can be waterproofed with paint, polyurethane, or epoxy finish. Lightly sand the plywood with 180 grit sandpaper, clean off any sawdust, and apply the finish with a brush or by pouring the finish on if using epoxy.
The least expensive plywood often has knot holes that should be filled prior to waterproofing. Otherwise, the change in height is an opportunity for water to get into the plywood. To fill the knot holes, press wood filler into the hole, slightly overfilling the hole, for the reason that wood filler tends to shrink as it dries.
Once the wood filler is dry – at least twenty minutes, but maybe an hour if it is a large hole – sand the wood filler down to match the surrounding plywood.
Sand the plywood with 180 grit sandpaper. Be extra careful around the edges; it is really easy to sand through the top veneer layer, which does not look great.
If you have a variable speed orbital sander, this is the time to use it on a lower speed to help keep away from sanding through the top layer. If your orbital sander is not variable speed, that is okay, just press lightly and keep moving. There are people who do not have a variable speed orbital sander, and they have never had an issue.
Once you have sanded the plywood, wipe it down with a damp cloth to remove any excess sawdust. You do not want sawdust to get stuck in your finish!
Finally, take into account that if you are painting your plywood, you can skip the sanding step altogether if you are using a bonding primer.
If you are planning to stain or prime the plywood before waterproofing, now is the time to do it.
You can easily find an entire post on staining plywood, but the general gist is to dip a rag into some stain, and spread the stain onto the plywood in the direction of the grain. This method is liked better than the foam brush method, since you will have more control over how much stain actually gets on the plywood.
Regardless of the method you choose, make sure to wipe off any excess stain within a few minutes of applying it to the piece. Wood stain does not dry, and just turns out to be a sticky mess if not wiped away.
Just to be extra clear: priming is a step you can do if you are painting the piece. It is not something you would do under any other finish. Staining is a step you would do under a clear finish to change the colour of the wood, but is unnecessary under painting.
Apply the waterproofing finish of your choice – paint, polyurethane, or epoxy. Paint and polyurethane can both be applied with a paintbrush or a roller. Epoxy resin is typically poured onto the plywood.
Polyurethane will have brush strokes or roller marks, simply because it is so thick. To get rid of these, sand these away after the last coat, then thin the polyurethane with mineral spirits so that you can wipe on final, thin coat.
This coat will not have brushstrokes because it is wiped on with a rag, and it will dry quickly so dust and dirt will not have time to get stuck in the finish.
Dry times vary between products. Paint usually can be recoated in about two hours, while oil-based polyurethane requires a full 24 hours between coats. You will need at least two coats of both paint and polyurethane.
Epoxy resin dry times vary based on the hardener used. Regardless, you will probably only need one coat of finish.
