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How to Seal a Deck With Thompson's Water Seal

Sealing Decks When They're New Relatively Painless

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Gallon can of Thompson's WaterSeal. I used Thompson's WaterSeal on my pressure-treated decking.

This gallon can of Thompson's Water Seal will cover 200-300 square feet of pressure-treated decking.

David Beaulieu

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 deck sealer 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 Web site, Thompson's Water Seal states a basic rule-of-thumb for choosing a deck sealer or stain: "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 colored, opaque shield to protect decks not only from moisture and mildew, but also from UV rays
  • you won't 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 2 extremes are "tinted finishes" and "semi-transparent finishes," among the Thompson's Water Seal products. For other brands of deck sealer, the corresponding 4 categories may be called "transparent," "semi-transparent," "semi-solid" and "solid." I used a Thompson's Water Seal product called "Waterproofer PLUS CLEAR Wood Protector" (photo) -- a clear finish.

Deck-Sealing Supplies

  • broom or leaf blower
  • 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. Sealing a new deck is a breeze -- assuming it hasn't received much use yet -- because it shouldn't 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. Much more work!

But even new decks, if they rest under messy trees, can require extra work to seal. In sealing my new floating deck, I was reminded once again that Eastern white pine trees, for all their beauty, can be quite a nuisance. Pitch from the pine tree that overhangs my deck fell on the decking and stained it with pitch, meaning I had to get down on hands and knees and scrub off pitch first before I could hose off my deck. I applied mineral spirits to a rag to remove the pitch.

But I wasn't through cursing the pine after removing the pitch. For the tree had also deposited pine needles and broken pieces of pine cone, which lodged between the cracks of my deck. To remove this debris, I had to 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

I applied Thompson's Water Seal to my deck using a product called "Twist N Reach," which consists of a paint roller on a telescoping pole. I tried this product in lieu of threading a paint roller on a pole (the typical approach), because I was attracted by the promise of convenience offered by the product's telescoping ability. But I was dissatisfied with the "Twist N Reach," as I found the telescoping mechanism unreliable (it was difficult to keep the joints tight, so the pole kept collapsing in on itself while I was working). In the future, I'll stick to the tried and true method of threading a roller on a pole. Some prefer to spray deck sealer on, using a junky old garden sprayer.

    When to Apply Deck Sealers
  • Surface and air temperature must be above 50 (Fahrenheit) during application and for 48 hours thereafter
  • Your forecast should indicate no rain for 24 hours after application

If you own an elevated deck and have anything under it that you wish to protect, now's 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 Thompson's Water Seal. Start rolling it onto the decking in the corner furthest away from where you'll be exiting the deck (if your deck has steps, then that's the point where you'll 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 I used called for only one thin coating. Leave one small area in front of your exit point uncoated. When you've covered all the decking except for that one small area, exit the deck and reach back to apply the Thompson's Water Seal there. Coat steps last.

Drying time is 48 hours. Clean roller, etc. with mineral spirits. When should you re-apply Thompson's Water Seal? 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 don't 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.

Related Video
Seal and Protect Your Deck
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