How to Waterproof Plywood
Now that we’ve discussed the different available methods of plywood waterproofing, it’s time to do the work of sealing it.
Step 1: Prepare the Plywood
Before the liquid latex, paint, or PVA is applied, the plywood needs to be readied. Clean the surface of the plywood sheet with a brush or dry cloth, removing any dirty spots you can find.
Then, use wood filler to fill in any rough spots on the surface of the sheet. This will give it a uniform smoothness and help prevent any gaps in the waterproofing. Give the wood filler time to dry and harden before proceeding to the next step.
Once the wood filler dries, wipe down the sheet with a wet cloth. That small amount of moisture will raise the wood’s grain a bit, helping the sealer bond to the plywood sheet.
The final step in preparation is to lightly sand the plywood with medium-grit sandpaper (60-100 grit). This will help smooth the sheet and remove any rough spots that could “gunk” up the painting/sealing.
Step 2: Apply the Sealer
If you’re using a brushed-on sealing method, paint it on with a brush or roller, just as you would normally. Make sure you get adequate coverage and let the first coat dry before applying the second. Some materials and waterproofing needs might lead you to add a third coat. While this might not be necessary, but it can never hurt!
If you’re using a spray method, carefully read the manufacturer’s instructions. Improper use of spray-on sealer results in one of two things:
not using enough sealer to cover the surface adequately, and
having a spotty concentration, with lots of sealer in some places and very little in others.
The reason these risks exist is that spray-on sealers are harder to see and control than brushed-on sealers. So, when you spray the sealer, be very intentional with where you are pointing the can!
Step 3: Let it Dry, and Keep an Eye
Give the plywood at least a few hours to dry, up to 24 hours if you have the time to spare. Then, once it’s completely dry, you’re free to install it! After plywood is installed, add a bit of sealer over the screws/nails to ensure it stays watertight!
Finally, although your plywood sheets are now waterproofed, you should still check them out every few months to make sure water isn’t wreaking havoc without your knowledge. Water damage will be easy to spot, so this shouldn’t feel like a comprehensive inspection- just an “eyeball test” to ensure all is well!
How to Seal Plywood Edges
We’ve gone over how to seal the surface of a plywood sheet, but what about the edges? Should they be waterproofed, and if so, how do you seal plywood edges? Because plywood is made of several layers, the edges are particularly vulnerable to water.
When water is introduced to plywood edges, it can split much faster than it would on the sheet’s surface. So, it’s very important to waterproof the edges- perhaps more important than surfaces!
Sealing plywood edges is relatively simple; apply several coats of varnish, allowing each coat to dry for about an hour. The edges will soak up the varnish quickly, so you need more coats to make sure it dries to form that protective layer.
Types of Waterproof Plywood
Plywood is graded on an A-D system; A is the highest and D is the lowest. The first letter represents the front face, and the second letter represents the back face and is usually more coarse.
The third letter is less important but signifies the type of glue used to bond the layers. These letters represent the appearance of the wood, not the strength it has. So, ABX looks much nicer than CDX, but they both have similar strength and water resistance. Let’s take a look at the different types of waterproof plywood and their recommended uses!
ABX Plywood
ABX, also known as marine-grade plywood, is a smooth, strong plywood. It’s intended for marine use, but it can also make great exterior plywood anywhere. It is specially treated to be resistant to salt and mildew along with water, and has the smoothest, most appealing surface of any plywood, hence the “AB” grade.
ACX Plywood
ACX is waterproof plywood that has much in common with marine-grade sheets but doesn’t have the marine-specific treatment. It is waterproof, tough, and perfect for use in decks, exterior walls, and renovations.
ADX Plywood
This type of plywood is pretty much the same as ACX, but the back face of the sheet is of the rougher, knottier variety. It can be a cheaper option because only the front face is the pricier “A” graded veneer. It’s great to use in exterior construction or as a subfloor in a home.
BWP Plywood
The letters in this plywood stand for “Boiling Water Proof”- and it’s exactly what it sounds like. This is extremely durable plywood and will keep its integrity even in the face of boiling water! It’s ideal to use when you are constructing a kitchen or bathroom where hot water and steam will be ever-present.
MR Grade Plywood
The “MR” stands for “moisture resistant” and is good indoor-use plywood. It isn’t completely waterproof- it resists moisture and water droplets but will falter against standing water. In humid climates, MR plywood is ideal for interiors because it won’t warp from humidity.
BWR Grade Plywood
“Boiling Water Resistant” plywood is like the slightly weaker cousin of BWR plywood. It will hold up well against steam and boiling water, although it is not technically watertight. It’s best used in places where water and steam might be present but not all the time- cabinets, sinks, etc. are all great places to use BWR grade plywood.
