How to Waterproof Wood

Waterproof vs water-resistant: what to expect

Almost no wood finish makes wood truly waterproof forever. Wood is a natural material that takes on and releases moisture, and it swells and shrinks as it does so. Most products sold as “waterproofing” are better described as water repellent or water resistant. They slow water absorption, shed liquid water and reduce the swelling, warping and rot that moisture can cause.

A thick, sealed coating (such as epoxy) can come close to blocking water, but it can fail at edges, joints or scratches, and once water gets behind a film it can be hard to dry out. So the goal is usually to choose a finish suited to the exposure and to keep it maintained.

Before you start: safety and prep

Finishing involves dust, solvents and flammable materials, so plan for safety first.

Ventilation. Work outdoors or somewhere with strong cross-ventilation. Keep away from pilot lights, sparks and open flames when using solvent-based products.
Respiratory and skin protection. Wear a dust mask or respirator rated for fine dust when sanding, and an appropriate respirator for organic vapors when using solvent-based finishes. Wear chemical-resistant gloves and eye protection. Follow the product label.
Oily rags. Rags soaked with oil-based finishes (linseed oil, many stains and varnishes) can generate heat as they cure and may ignite on their own. Lay them flat to dry outdoors, away from buildings, or store them in a sealed metal container filled with water, as the manufacturer directs.
Power tools. If you use a sander, wear hearing and eye protection, keep cords clear and use dust extraction where you can.

Preparation matters as much as the product:

Clean off dirt, mildew, grease and old loose finish. Let the wood dry fully; a damp piece can trap moisture under the coating.
Sand to a surface the product can grip. Check the label, because very fine sanding can make some penetrating finishes absorb poorly.
Remove dust with a vacuum and a clean cloth or tack cloth.
Seal end grain. End grain soaks up water far faster than faces and edges, so give cut ends, tenons and the bottoms of posts extra coats.

Waterproofing methods compared

Penetrating oils and waxes

Oils such as tung oil, linseed oil or modern oil blends soak into the wood. They give a natural look and are easy to refresh, but they usually provide modest protection on their own against sustained outdoor weather. Some raw oils can feed mildew outdoors, so exterior-specific products with mildewcides and UV inhibitors are generally a better choice. Waxes add surface water beading but wear off quickly.

Water-repellent sealers

Clear or lightly tinted water-repellent sealers are designed for decks, fences and other outdoor lumber. They penetrate rather than form a thick film, so they tend not to peel, and recoating is usually a matter of cleaning and reapplying. Look for products stating UV protection if the wood will be in sun. Our wood sealer vs. stain article explains how sealers differ from stains. For decks specifically, see our guide to maintaining a treated wood deck.

Spar varnish and exterior urethane

Spar varnish (also sold as spar urethane) is formulated for exterior use. It builds a flexible film and typically contains UV absorbers, which helps on doors, furniture and railings. Because it is a film finish, it needs periodic maintenance: if it begins to crack or peel, water can get in under it. See our guide to clear finishes for exterior doors for how this applies in practice.

Standard interior polyurethane is designed for indoor use and often is not suited to sun and weather exposure. For more on varnish types, see best varnish for wood.

Epoxy

Epoxy resin forms a hard, plastic-like layer and is used on boats, river tables and items in constant contact with water. It is generally not UV stable on its own and can yellow or degrade in sunlight, so it is commonly topped with a UV-resistant clear coat. Epoxy fumes and skin contact need serious caution: use gloves, eye protection, ventilation and follow the manufacturer’s mixing and cure instructions exactly.

Paint and solid stain

Opaque paints and solid stains form a barrier and block UV effectively. They can protect weathered wood well but hide the grain, and if a film coating fails it can peel. Our comparison of deck paint vs stain walks through that trade-off.

Can you spray something on wood to make it waterproof?

Yes. Many water-repellent sealers are available in spray form, and spraying can be convenient for fences, furniture and rough-sawn surfaces. Spray in calm conditions, protect nearby plants and surfaces, and wear a respirator suitable for the product, because airborne mist is easy to inhale. Back-brushing or wiping after spraying can help the product work into the wood. Spray finishes and rags should be treated as flammable as well.

Choosing by project

Deck and fence: a penetrating water-repellent stain or sealer, or a solid stain or paint if the surface is weathered. Pressure-treated boards are covered in is tanalised wood waterproof.
Plywood and sheet goods: see is hardwood plywood waterproof before relying on a finish alone.
Entry door and exterior furniture: spar varnish or an exterior water-based clear finish for a clear look; paint if you want the most coverage.
Cedar and other naturally durable wood: penetrating, non-film exterior finishes are commonly recommended; see finishing cedar wood.
Planters and raised beds: avoid anything not labeled safe for contact with food plants if growing edibles; check the label.
Cutting boards and food contact items: use only finishes labeled food safe, and do not use exterior varnishes.
Wet or marine use: epoxy plus UV-resistant topcoat, following the manufacturer’s system.

Applying a finish step by step

Check the forecast. Most exterior finishes need dry weather and moderate temperatures; the label lists the range.
Test on a hidden area or offcut for color and absorption.
Apply thin, even coats along the grain with a brush, pad or rag as directed. Thick coats can dry poorly and fail early.
Wipe or back-brush excess from penetrating products as instructed so it does not pool and form a sticky film.
Allow the recoat time on the label and lightly sand between coats of film finishes.
Let the finish cure before exposing it to heavy water or use.
Dispose of rags safely, as described above.

Maintenance and recoating

Water beading is a handy gauge: if water soaks in or darkens the wood instead of beading, the finish is wearing. Penetrating sealers are commonly renewed every year or few years depending on exposure, and film finishes should be touched up at the first sign of cracking or peeling. Cleaning before recoating helps adhesion.

Common mistakes

Finishing wood that is still wet.
Skipping end grain and the underside of boards.
Using an interior finish outdoors.
Applying film finishes too thickly.
Waiting until a film has failed badly, which often means stripping.
Leaving oily rags in a pile.

FAQ

What is the best way to waterproof wood? It depends on the project. Penetrating water-repellent sealers suit decks and fences, spar varnish suits doors and furniture, and epoxy with a UV topcoat suits constant-water uses.

What do you put on wood to make it waterproof? Common choices are water-repellent sealers, exterior stains, spar varnish, epoxy systems, paint and, for some items, oils or waxes. Choose one labeled for your exposure.

Can wood be made fully waterproof? Not reliably over the long term. Aim for water resistance and maintain the finish.