5 Ways to Seal/Waterproof Plywood for Outdoor Use

By MWB-Team •  Updated: 06/16/23 • 

Plywood is one of the most affordable and versatile construction materials around. It can be used for both interior and exterior construction. However, when used for the latter, you’ll need to seal or waterproof it to prevent warping or decay when exposed to different outdoor elements.

There are different ways of sealing or waterproofing plywood for outdoor use. In this article, we’ll walk you through the different methods and a step-by-step guide to waterproofing and protecting it in an outdoor setting. You can also check out our guide on waterproofing furniture for outdoor use.

How to Seal/Waterproof Plywood

Using Polyurethane Varnish

Polyurethane is a thin varnish that you can use on plywood to make it waterproof. Common applications of polyurethane include balconies, roofs, kitchens, patios, kitchens, and wet areas. The main advantages of using a polyurethane varnish sealer are you get a clear protective coat, and it’s easy and fast to apply.

When choosing your waterproofing polyurethane varnish, always check if it is recommended for outdoor or exterior use. Also, we’d suggest using an oil-based poly varnish than a water-based poly varnish. While you can get away with using water-based polyurethane varnish, the oil-based poly varnish is more durable. It offers better protection because of its penetrative nature.

Using Liquid Latex/Rubber

Liquid latex or rubber is another great method used for waterproofing different surfaces. It is easy to apply, and you can brush it or use a good paint sprayer. When using liquid latex/rubber, the key is applying heavy coats to build up a waterproof membrane. Once applied to your plywood, it will take around 24-48 hours to cure.

The advantage of using liquid latex is that it is water-based and thus contains no solvents, volatile organic compounds (VOCs), or harmful odors. It is also safe for pets and humans alike.

Although it has many advantages, liquid latex or rubber may need a primer sometimes and is expensive compared to other options in this list.

Using Epoxy Sealer

Epoxy sealer provides a strong, thick waterproofing coat when applied to plywood. Epoxy is mostly used for sealing garage floors and finishing most wood products. Applying epoxy on plywood will be laborious, time-intensive, and expensive. However, once you’re finished, the results are worth it.

Epoxy sealers will take a bit of preparation before application. First, properly mix the hardener and epoxy resin using the manufacturer’s instructions. Also, ensure the conditions are right – avoid colder and high-humidity weather, which can interfere with curing.

Epoxy can be applied to your plywood using a roller or squeegee. Wait until the first coat feels tacky (3-4 hours) before applying a second coat (2-4 coats are enough). For better protection against elements like UV, you can opt to use a colored topcoat to ensure a lasting finish.

Using Oil

The use of oils for waterproofing is recommended for lighter waterproofing, where you’re protecting plywood against moisture. There are different types of penetrating oils you can use for waterproofing. Tung oil, Boiled Linseed oil (BLO), and mineral oil are some of the best.

Among the three, tung oil is more water resistant and creates a harder finish, while BLO and mineral oil offer cheaper alternatives. You can also combine different oils, however, make sure each oil soaks in before applying a different one.

Unlike other waterproofing methods, oils penetrate the wood fibers and saturate them. This leaves no room for water absorption and repels water on the surface of the plywood. Let the oil sit in longer to saturate the wood fibers.

Using Stain Sealant

This is a good method of waterproofing or sealing plywood if you want to keep the look of the wood. A stain sealant adds color to the plywood and creates a waterproof seal.

There are two types of stain sealants to choose from, and these are oil- and water-based stain sealants. Oil-based stain sealants penetrate deeper into the wood fibers to provide better weatherproofing resistance. However, they contain more VOCs and can take longer to dry. Water-based stain sealants sit on the surface but provide good waterproofing and UV resistance.

Apart from water- and oil-based stains, you can choose the opacity of your stain sealant from transparent, semi-transparent, and solid or opaque.

Step-by-Step Guide to Seal/Waterproof Plywood

Step 1 – Set Up a Clean Workspace

Before you can start working on your plywood, first set up a clean workplace. Find a good outdoor area and set up a flat space to lay your plywood while working on them. Several benches can provide a clean and stable area to work on. You can also lay the plywood on flat ground, but remember to add old newspapers or a clean surface to lay it on.

Step 2 – Apply Wood Filler/Putty to knots and Holes

The first step is to clean off any debris on the plywood’s surface and fill knots and holes with a wood filler/putty. Apply a small wood putty or filler to the knots and holes, scrape away the excess, and let it dry off.

Step 3 – Sand it Down

Once you’ve filled the knots and holes and it s dry, sand them down to leave a smooth plywood finish. Sanding will also help sealers and waterproofing agents penetrate deeper into the wood fibers. Use 180 to 220-grit sandpaper, working along the grain of the plywood.

Step 4 – Apply Your Sealer/Waterproofing Coat

Once that is done, wipe the plywood with a damp cloth in preparation to seal or waterproof the surface. Depending on your chosen waterproofing method, apply it and let it cure before applying the second coat. Ensure the edges are properly sealed because they allow moisture to be absorbed.

MWB-Team

Hello there! This is the Make Wood Better Team. Here, we share informative how-tos and guides focused on making wood better. Whether it's finishing wood, maintenance, or restoration, there's something on this website for you to learn and improve your skills.