A Guide on How to Seal Decking

Protecting the Boards While They’re Still New Is Relatively Painless

Thompson’s Water Seal is one of the deck sealers commonly used to protect decks from such menaces as moisture and mildew. If you just had a new deck built, you probably realize that you need to coat it with a sealant to protect it from the elements. But whether you choose to use Thompson’s Water Seal or another brand, you may feel overwhelmed trying to decide between all the options available.

At the heart of the learning process is differentiating between deck sealers and stains. On their website, Thompson’s Water Seal states a basic rule-of-thumb for choosing between these two related but different products, which is built on the following observation: “the more wood grain you want to see, the more often you will have to re-coat your deck  over time.” Thus, at one end of the spectrum, we find “clear finish” among the Thompson’s Water Seal products:

    • Clear finishes allow you to see the natural wood grain.
    • But you will have to re-coat more often with clear-finish deck sealers, because that very transparency renders them less effective at blocking UV rays.
    • Clear-finish deck sealers do protect against moisture and mildew.
    • Such a product is an example of a true deck “sealer.”

At the other end of the spectrum for Thompson Water Seal products, we find “solid finish”:

    • Solid finishes create a coloured, opaque shield to protect decks not only from moisture and mildew, but also from UV rays.
    • You will not have to re-coat your deck as often, but nor do they allow you to appreciate your deck’s wood grain.
    • Such a product is an example of a true deck “stain.”

In between these two extremes are “tinted finishes” and “semi-transparent finishes,” among the Thompson’s Water Seal products. For other brands of deck sealer, the corresponding four categories may be called “transparent,” “semi-transparent,” “semi-solid” and “solid.” A Thompson’s Water Seal product called “Waterproofer PLUS CLEAR Wood Protector”—a clear finish—was used in the project referenced below.

Supplies That You Will Need 

    • Broom or leaf blower
    • Garden hose
    • Mineral spirits, rag, screwdriver
    • Roller, pole
    • Paint brush
    • Large pan
    • Thompson’s Water Seal
    • Protective clothing, gloves, goggles

Cleaning the Deck Before Applying Deck Sealer 

Deck sealers can be applied to new decks 30 days after construction, according to some experts (others recommend waiting for a longer period). Sealing a new deck is a breeze — assuming it has not received much use yet — because it should not be very dirty. Just sweep off any debris (or blow it off with a leaf blower) and hose it off with a garden hose. By contrast, when preparing to seal an older deck, it is often recommended that you use a pressure washer first to remove deep-seated grime. This entails much more work.

But even new decks, if they rest under messy types of trees, can require extra work to seal. You will become acquainted with this fact if you are unfortunate enough to be treating a deck that rests under an Eastern white pine tree. Such trees, for all their beauty, can be quite a nuisance. Pitch from the pine tree that overhangs your deck will fall on the decking and stain it with pitch, meaning that you will have to get down on hands and knees and scrub off pitch first before you can hose off your deck. Apply mineral spirits to a rag to remove the pitch.

Even after completing this pitch clean-up task, though, you may not be through cursing the pine tree. For your tree may have also deposited pine needles and broken pieces of pine cone, which will lodge between the cracks of your deck. To remove this debris, run a screwdriver down the length of each crack.

After hosing off the deck, wait 48 hours for it to dry before applying the deck sealer — unless you can find a Thompson’s Water Seal product called “Advanced Natural Wood Protector,” which requires no waiting period between cleaning and application.

Applying the Deck Sealer 

One way to apply Thompson’s Water Seal to a deck is to use a product called “Twist N Reach,” which consists of a paint roller on a telescoping pole. You can try this product in lieu of threading a paint roller onto a pole (the typical approach). Indeed, you may be attracted by the promise of convenience offered by the product’s telescoping ability. But you may experience dissatisfaction with the “Twist N Reach.” The telescoping mechanism is unreliable (it is difficult to keep the joints tight, so the pole tends to keep collapsing in on itself while you are working). Stick to the tried and true method of threading a roller onto a pole. Alternatively, some prefer to spray deck sealer on, using a junky old garden sprayer.

When should you apply deck sealers (in terms of weather conditions)? Here are two rules of thumb:

  • Surface and air temperature must be above 50 degrees (Fahrenheit) during application and for 48 hours thereafter.
  • Your forecast should indicate no rain for 24 hours after the application.

If you own an elevated deck and have anything under it that you wish to protect, now is the time to cover it with a tarp. Otherwise, the Thompson’s Water Seal may drip down on it, through the cracks in the decking.

To begin, pour some of the liquid from the Thompson’s Water Seal can into a large pan (or a similar container that the roller will fit into easily). Seal deck railings first, using a paint brush. Now thread the roller onto its pole and dip it into the pan to pick up some of the sealant. Start rolling it onto the decking in the corner furthest away from where you will be exiting the deck (if your deck has steps, then that is the point where you will most likely be exiting).

Gradually work your way back to the exit as you spread the Thompson’s Water Seal evenly across the decking. The label on the product calls for only one thin coating; indeed, some have learned the hard way that two coats of deck sealer are not necessarily better than one (see below). Leave one small area in front of your exit point uncoated. When you have covered all of the decking except for that one small area, exit the deck and reach back to apply the Thompson’s Water Seal there. Coat the steps last.

Drying time is 48 hours. Clean the roller, etc. with mineral spirits. When should you re-apply the sealant? Gauge the need for future applications by conducting a “water test,” which simply entails splashing water on the decking and observing the results. If the water beads up, you do not yet need to re-seal. But if the water seeps right into the wood, the seal is broken, and you need to re-apply Thompson’s Water Seal.

Case Study: Two Coats Are Not Better Than One 

In some cases, in life, having two of a particular thing is clearly better than having just one. Thus, the old expression “Two heads are better than one.” But does this maxim apply to using deck sealants? If one coat is good, would two coats be better?

We all know how important it is to preserve the wood of a deck with a stain or sealant. Wood is, after all, a biodegradable material and, if left to fend for itself, will begin to break down over time.

As proof, just observe what happens to fallen logs in wet forests: They eventually rot, returning to the soil from whence they initially sprang as seedlings. The pounding sun is another natural enemy of decks. Homeowners must take action to preserve their investment. To serve this need, various deck sealants have sprung up on the market; brands include:

      1. Thompson’s (see above)
      2. Ronseal
      3. Osmo
      4. Cuprinol
      5. Valspar

But will two coats of deck sealant do a better job than one? According to reader, Daniel from Rockland, Ontario, the short answer is, “No.” He tells us why he reached this conclusion by recounting his own experience in applying a deck sealant:

“Regarding your article about sealing a deck with Thompson Water Seal, I wanted to relate my own experiences using a product called Thompson’s Water Seal Plus Clear Stain. In a nutshell, I found that applying just one coat yielded a far superior result to using two coats. Here’s my story:

“I was told by the deck contractor that I had to wait one full year before applying any sealants or stains.

One year after they built it, I started using a power washer to clean my deck, and a deck sander to sand it. Then I followed the instructions that were provided to me:

“To prevent the deck from peeling a year later, I had to wash the deck with dish soap, rinse, and use a squeegee to remove water on it. When I started the first coat of Clear Plus stain, I noticed the liquid was sort of a lime colour, not clear like a glass of water. After I finished the first application and let it dry for 48 hours, I did notice where the knots were in the pressure-treated lumber: it was whiter compared to other places on deck, suggesting (I thought) a second coat should be applied, one month later.

“That second coat turned out to be a big mistake. After the second coat, I noticed (48 hours later) it was still wet in some places, therefore I was forced to hose the deck down with fresh water, as I had a pool party scheduled for the next day. I rinsed it, then used a squeegee to remove water on it. I noticed three things about the deck that were now different from when it had had just the one coat:

      1. The deck squeaks almost everywhere that you walk on it.
      2. When it’s sunny and over 30 degrees (Centigrade) outside, the deck is too hot to walk barefoot on. This makes me an unhappy customer on hot days.
      3. Most importantly, the second coat made the deck much darker, even though I used a clear-finish product.”

Thanks for that story, Daniel. You will hear different advice regarding whether you should apply one or two coats, some of it product-dependent (following the instructions on the container is the easiest way to settle the question). In a sense, the issue isn’t really whether a second coating is good or bad, in and of itself. What matters is the amount of deck sealant being applied.

What’s the correct amount? You should be using enough sealant so that water beads up on the decking when it rains (in fact, you should conduct what’s called a “water test” — to determine whether a previously sealed deck needs to be sealed again — by spilling some water onto your deck and seeing if it beads up). It really doesn’t matter whether it takes one or two coats to achieve this objective.

Keep in mind, though, that there is such a thing as “too much sealant.” All of the sealant should be absorbed into the wood. Excess sealant will peel. So, if a second coat fails to penetrate the wood, then, yes, that would be a problem. Likewise, thin coatings are better than thick ones, because it’s easier to add to a thin one than to subtract from a thick one. Remember, too, that circumstances play a role in determining what’s the right amount of sealant to use. A severely weathered deck will drink in a considerable amount of sealant before its thirst is quenched.

Incidentally, a type of deck called a “floating” deck is the easiest kind for a beginner to build. Are you game? Read these instructions on how to build a floating deck.

By David Beaulieu