Deck sealer vs stain vs waterproofing: what each does and which your deck needs

At the hardware store, one can says sealer and the next says deck waterproofing. On a wooden deck they do much the same job in different amounts: they slow the water down, and a stain also blocks the sun. Pick by how much colour you want, how often you’re willing to recoat and what’s already on the boards.

We do this work across Markham through our deck staining, sealing and waterproofing service, and this is the answer we’d give you standing on your deck.

What is the difference between a deck sealer and a stain?

A sealer is clear and a stain has pigment. That’s the main difference.

A clear deck sealer is usually a water repellent: an oil or varnish plus a wax, in a solvent that carries them into the wood, according to the USDA Forest Products Laboratory (FPL). Water beads on the surface instead of soaking in.

The better ones are water-repellent preservatives, which add a mildewcide. The FPL says those perform much better, and that plain water repellents often leave the wood blotchy.

A stain adds pigment. The pigment shades the wood and the mildewcide from sunlight, which the FPL says is why stains last longer.

Between clear and opaque you’ll find:

• A toner, sold as a coloured or tinted sealer, has a little pigment and most of the grain shows.
• An oil-based semi-transparent stain has a lot more pigment but still soaks in without forming a layer, so it won’t blister or peel. Waterborne versions are more likely to leave a film.
• A solid stain sits on top as a film, hides the grain and behaves more like paint.

The FPL recommends penetrating finishes for deck floors: sealers, toners and semi-transparent stains.

Film finishes like paint and solid stain trap moisture, wear through where people walk and fail by peeling.

Does deck waterproofing actually make wood waterproof?

No. The FPL’s handbook on exterior finishes (Agriculture Handbook 647) says no coating is ever entirely moisture-proof, however many coats you put on. It only slows the rate at which the wood takes on and loses water.

On a deck that still counts for a lot, because the constant swelling and shrinking is what opens up checks and cracks.

So when a can says waterproofing, read it as water repellent. Thompson’s, for example, sells its clear sealer on protection from water and sun damage, then recommends a fresh coat every year.

True waterproofing is a different product for a different deck. If the deck sits over a room or you want a dry patio underneath, you need a membrane over a solid plywood deck, such as vinyl decking or the elastomeric coatings Handbook 647 describes.

Duradek, for one, calls its vinyl roof-grade waterproof protection with a dry-below system. It recommends untreated tongue-and-groove plywood underneath and only backs it with a written warranty when an authorized Duradek contractor installs it. On a regular board deck, water is meant to drain through the gaps.

How long does each deck finish last?

On a deck floor, more pigment means a longer life, but film finishes trade that for peeling. Here’s what the sources say about each type on horizontal deck boards:

Consumer Reports bases its figures on up to three years of testing: pine boards facing south and angled like a roof, on top of its headquarters in Yonkers, New York.

Which finish does your deck need?

Start with what’s on the boards now, then decide how much colour you want.

1. New pressure-treated wood. Sprinkle water on it. If it soaks in, a clear sealer or semi-transparent stain can go on. Thompson’s asks for 30 days on new pressure-treated lumber, and the FPL says decking with a factory water repellent may need 2 to 3 months of weathering first.
2. Grey, weathered wood with no finish left. A semi-transparent stain is usually the right call. The FPL says a deck kept up with clear sealer for years can switch to a semi-transparent stain and do fine.
3. You’re unsure. The FPL’s own advice is to use a clear water-repellent preservative first. It’s the easiest finish to reapply, and you can move to a stain next time.
4. Solid stain or paint already on the floor. A penetrating product can’t soak through it. You either keep recoating with a film finish after scraping the loose stuff, or strip back to bare wood and switch.
5. Railings and spindles. They don’t take foot traffic, so the FPL says paint and solid stain can do well there, even with a penetrating stain on the floor.

Fences raise the same choice, minus the foot traffic. Our fence painting vs staining guide covers them.

Can you put sealer over stain, or stain over sealer?

Not right away. A penetrating sealer has to soak into bare wood, and Thompson’s says not to use its clear sealer on painted or recently stained surfaces.

Going the other way, the FPL says a semi-transparent stain may not absorb into wood recently treated with a water-repellent preservative, and suggests letting it weather for 2 to 3 months.

Cans sold as stain and sealer in one can be anything from a toner to a solid stain, so check the label for how much colour it has. Handbook 647 notes that some penetrating stains already contain a water repellent.

Can you waterproof a deck in October?

Usually not, and that catches a lot of people out, because October is when deck waterproofing gets searched most. The problem is the nights.

Thompson’s asks for the surface and air to stay above 10 °C for 48 hours: while you apply it and for 48 hours afterwards. It also wants no rain forecast for 24 hours.

At the Buttonville station in Markham, Environment Canada’s 1981–2010 normals give October an average low of 3.6 °C and about 7 nights at or below freezing.

If your deck is soaking up water this fall, here’s what to do instead:

1. Wash it, gently.
2. Clear the leaves off so the boards can dry over winter.
3. Plan the coat for late May to early September.

Our deck staining guide has the month-by-month weather table and the drying rules.

What are the most common sealing and staining mistakes?

Most failures come from prep and timing rather than the product:

• Coating wet wood. The FPL says to allow at least 2 warm, windy days of drying after cleaning with a bleach solution.
• Putting clear sealer on and expecting the colour to last. Without pigment, little stops the sun greying the wood.
• Laying it on thick. Penetrating finishes are meant to soak in, and the FPL notes that some sealers leave the deck oily until they absorb.
• Rolling a semi-transparent stain over old solid stain or paint. Handbook 647 says penetrating stains aren’t effective over either.
• Power washing too hard. The FPL warns that aggressive scrubbing or power washing removes wood from the surface, especially on soft cedar. Our guide to pressure washing decks without damage has safer settings.
• Lap marks. The FPL notes that overlapping semi-transparent stain onto a section that’s already done leaves darker stripes. Work a few boards at a time along their full length.

When is it worth calling someone?

A sound deck with no old finish is a fair DIY job if you can wait for the weather.

We’d suggest getting help when the old finish is peeling and has to come off, when boards are loose or soft, or when there are long stair runs and dozens of spindles. Prep is most of the work on those decks, and it decides a lot about how long the new coat lasts.

Our deck restoration project shows the kind of job we mean: a grey, weathered deck with a long staircase, pressure washed and then given a waterproof coating.

If yours looks like the before photo, call or text (437) 684-2535 with a couple of photos, or see our deck staining and waterproofing service. For published price ranges and worked examples, read how much deck staining costs.