How to Seal Painted Wood for Outdoor Use

Colorful Painted Outdoor Set of Chairs

mmac72 / Getty Images

Project Overview
  • Working Time: 1 - 2 hrs
  • Total Time: 1 - 3 days
  • Skill Level: Beginner
  • Estimated Cost: $20 to $40

No place is harsher for painted wood than the outdoors, but additional coatings and sealants can counteract some of the exterior's detrimental effects. Sealing painted wood is simple and quick, and it can help preserve the wood's appearance and prolong its lifespan.

Why You Should Seal Painted Wood

Sunlight's ultraviolet (UV) rays are the main enemy of painted outdoor items such as chairs, tables, play equipment, and more.

Photodegradation breaks apart the molecules that form paint's tight, protective bond. This destructive process also has an avenue through paint's pigments—the solids that create paint color—which absorb UV rates at a greater rate and speed up the decay.

Add to that temperature fluctuations, moisture, and wood's tendency to expand and contract, and you've got a rapidly ticking clock until paint loses its color, elasticity, gloss, and adhesion.

Painted wood surfaces located outdoors must either be taken indoors—often, not an option—or protected. Protection is the work of wood sealers, which slow down paint decay.

When You Should Not Seal Painted Wood

Painted wood that's been coated with exterior-grade paint does not need to be sealed with a clear coat. Two or more coats of exterior paint are already sufficient outdoor protection. As long as the paint and underlying wood are in good condition, the paint can remain as-is.

The best sealer for wood painted in exterior-grade paint is another layer or two of exterior-grade paint. No clear coatings are necessary.

If you do decide to clear-coat painted wood, know that some oil-based and latex paints can turn an amber color under polyurethane coatings. First, test the sealer on an inconspicuous section of the painted surface to see if the color changes.

When to Seal Painted Wood

There are a few conditions when you might want to apply a clear sealer instead of exterior-grade paint over painted wood:

Safety Considerations

Wear eye protection when working with TSP and polyurethane sealers. Work in well-ventilated areas when sealing the wood, especially with oil-based coatings.

Brushes used with oil-based sealers must be cleaned with paint thinner or mineral spirits. As these products contain powerful chemicals, take care when using them. Wear goggles, work in ventilated areas, and properly dispose of waste materials with community-approved methods.

What You'll Need

Equipment / Tools

  • Paint brush
  • Shop vacuum with brush attachment
  • Tack cloth
  • #220 and #320 grit sandpaper
  • Electrical oscillating sander
  • Clean bucket
  • High-quality sponge or lint-free rags
  • Rubber or latex gloves

Materials

  • Polyurethane sealer (water- or oil-based)
  • Trisodium phosphate (TSP)
  • Paint thinner or mineral spirits (for oil-based sealers)

Instructions

  1. Clean Painted Wood

    Mix the TSP with warm water in the clean bucket. Wearing latex or rubber gloves, soak the sponge or rag in the TSP solution. Squeeze out the sponge or rag until no more water comes out.

    Lightly wipe down the painted wood to remove oils, dirt, and other heavy residues. Be careful not to soak the surface.

    Tip

    Many communities have banned phosphate-based detergents—a class that includes TSP. If this is the case in your area (or if you just want to avoid phosphates), look for a product called TSP substitute. Usually found in liquid form, TSP substitute mixes easily with water and is safe for the environment.

  2. Lightly Sand Painted Wood

    After the surface has completely dried (about one day), lightly sand it with the oscillating sander fitted with fine-grit #220 sandpaper. Latex paint does not sand well. If you press too hard, you risk pilling up and sloughing off the paint. So, go easy.

  3. Remove Dust

    First, clean off the majority of the dust with the shop vacuum and brush attachment. Follow by wiping down the painted wood with the tack cloth. Do not press hard on the tack cloth. This can force wax into the surface, necessitating another round of sanding and cleaning.

    What Is Tack Cloth?

    Tack cloth is cheesecloth impregnated with wax. It's used like a damp cloth for cleaning. But because it's dry, it doesn't leave water behind to damage the surface.

  4. Apply Polyurethane Sealer

    Wearing eye protection, apply the polyurethane sealer to the painted wood surface with the brush.

  5. Sand Cured Polyurethane

    After the sealer has cured to a hard finish, lightly sand the surface with #220 sandpaper on the electric sander. Follow with a pass with the #320 grit sandpaper.

    Tip

    Water-based polyurethane dries to a hard coat in one to three hours. Oil-based polyurethane dries in one to two days per coat.

  6. Remove Polyurethane Dust

    Use the vacuum and tack cloth to remove the polyurethane dust from the surface.

  7. Apply More Coats of Polyurethane Sealer

    At least one more layer of sealer will be needed for durability and adhesion. Between each coating, sand and clean the surface.

Article Sources
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  1. Why Does the Sun Damage Paint? / Paint & Coatings Industry Magazine