How to Seal a Deck With Thompson's WaterSeal

After a good cleaning, seal your deck with one coat of sealer or stain

Backyard deck
Chuckcollier/Getty Images

Thompson's WaterSeal includes a complete line of deck sealers and stains for protecting deck wood from moisture and sunlight. Applying a sealer and/or stain typically is recommended every one to three years, depending on the finishing product, deck use, weather conditions, and sun exposure. All Thompson's WaterSeal products are DIY-friendly, and most need only a single application on a clean, dry deck. You can apply some products with a garden sprayer or paint roller, but for classic deck stain, a paint pad on an extension pole is best.

Finishing your deck starts with choosing a sealer or stain/sealer. Generally speaking, a sealer does not color the wood and is designed primarily for protection against moisture, while a stain/sealer includes pigment that helps protect against the damaging effects of sunlight (UV degradation) in addition to sealing out moisture. There are three basic categories of sealers or stains: clear sealer, semi-transparent stain, and solid stain. Keep in mind that all deck stains are also sealers.

Clear sealer offers very little UV protection but retains the natural color of the wood. It is not recommended where decks are subject to significant sun exposure. It also must be reapplied more often than most stains, typically every year. Semi-transparent stain adds some color to the wood and offers a moderate amount of UV protection. It is the most common type of deck finish because it strikes the best balance between UV protection and a natural wood look. Solid stain is similar to paint, as it hides the wood's grain almost completely. The tradeoff is that solid stain offers the best UV protection.

When to Seal Your Deck

There are a few rules of thumb dictating when you should apply a deck sealer or stain:

  • For clear sealer, the deck surface and air temperature must be above 50 degrees F during application and drying time.
  • For deck stain, the deck surface and air temperature should be between 50 and 95 F for application and drying.
  • The forecast should indicate no rain for at least 24 hours after application.
  • The deck surface should be allowed to dry for at least 48 hours before application.

It's also a good idea to wait for a calm day (little or no wind) to finish your deck, especially if you plan to use a garden sprayer. Stain is best applied when the deck surface is not in direct sunlight.

Project Metrics

Working Time: 1 to 2 days

Total Time: 4 to 5 days

Skill Level: Beginner

Material Cost: Between $17 and $30 per gallon of sealer or stain

What You'll Need

Equipment/Tools

  • Broom
  • Garden hose with spray nozzle
  • Tarp or plastic sheeting
  • Garden sprayer
  • Synthetic-bristle scrub brush
  • Synthetic-bristle deck brush
  • Hammer and nail set or drill
  • Utility knife
  • Sandpaper
  • Painting pad or paint roller and tray
  • Extension pole
  • Paintbrush
  • Rags

Materials

  • Deck cleaner
  • Thompson's WaterSeal deck sealer or stain
illustration of how to apply deck sealer
Illustration: © The Spruce, 2019

Instructions

  1. Prepare the Area

    Spray plants, grass, and other vegetation below or near the deck with water. Cover the plants with a tarp or plastic sheeting to protect them from the deck cleaner. Also cover windows, walls, or other surfaces directly below the deck to protect them from dripping sealer or stain.

  2. Prepare the Deck for Cleaning

    Sweep the deck thoroughly to remove all loose dirt and debris. Shake or stir a deck cleaner product, such as Thompson's 3 in 1 Wood Cleaner, as directed by the manufacturer. Add cleaner to a garden sprayer.

  3. Clean the Deck Railings

    Spray a small section of deck railing and balusters with deck cleaner, using the garden sprayer. Plan to clean small areas at a time so that you can scrub and rinse all of the wood before the deck cleaner dries. Do not let cleaner dry on the wood. Let the cleaner sit for 10 to 15 minutes, or as directed. Scrub the railing and balusters with a stiff, synthetic-bristle scrub brush to remove dirt and discoloration. Immediately spray the area with clean water, using a garden hose and sprayer. Repeat to clean the rest the of the railing.

  4. Clean the Deck Boards

    Clean the deck boards, using the same techniques used for the railings, cleaning about 10 to 15 square feet at a time. Scrub the boards with a long-handled synthetic-bristle deck brush. Work backwards from one far corner of the deck, finishing at a door or stairs so you can exit the deck without walking over the cleaned areas. Let the deck dry for at least 48 hours. If you covered grass or other plants, remove their protective covers while the deck dries, to prevent killing the plants.

  5. Check for Loose or Raised Fasteners

    Inspect the entire deck for loose or raised nails or screws, and drive the fasteners below the surface of the wood with a hammer and nail set or a drill. Also remove splinters or other potential hazards, using a sharp utility knife or sandpaper. Wipe away all sanding dust thoroughly. You can perform this step while the deck is drying after cleaning.

  6. Seal or Stain the Railings

    Apply sealer or stain to the deck railings, following the product directions; application methods vary slightly by product. A paint pad works well for the broad faces of railing and balusters, while a paintbrush is best for corners and tight spots. You can also apply a sealer or stain with a garden sprayer. Apply an even coating of sealer or stain to all surfaces, working in small areas and maintaining a wet edge (do not let the sealer or stain dry before moving to the next area). After completing each area, inspect the finish for drips or pooling, and redistribute or wipe up all excess with your application tool or a clean rag.

  7. Seal or Stain the Deck Boards

Apply sealer or stain to the deck boards, using the same techniques used on the railings and your choice of applicator: paint pad, paint roller, paintbrush, or garden sprayer. Apply the finish to 10 to 15 square feet of boards at a time, then wipe up any excess with a rag or your applicator. Work from the far corner of the deck to an exit point. It's usually best to cover a few boards at a time, working down their entire length, then working on the next few boards in the opposite direction. Let the sealer or stain dry for 24 to 48 hours (or as directed), depending on the product and conditions, before using the deck.

Dispose of Rags Safely

Deck sealers and stains may contain solvents that can cause rags to spontaneously combust. Dispose of soiled rags in a sealed, water-filled container or through another safe disposal method. Oily rags left in direct sunlight can start on fire in a matter of minutes.

Deck Sealing and Staining Tips

Thompson's WaterSeal products typically need only one application, but you can apply a second coat of stain to darken the wood, if desired. However, you must wait for the first coat to dry (typically about 2 hours) and apply the second coat within 4 hours of applying the first coat.

Follow the manufacturer's recommendations for refinishing your deck every one to three years, as needed. Deck wood stays in much better shape if it is regularly maintained. Letting it go for an extra year or two usually results in permanent damage that shortens the life of the wood.