Do I need to wash exterior wood before painting?

Washing exterior wood before painting is an important preparatory step that helps ensure the new paint adheres properly and lasts longer. Cleaning removes dirt, mildew, chalky paint residue, and other contaminants that can cause peeling or alligatoring if painted over. While pressure washing is ideal for cleaning large areas, other methods like detergents, cleansers, or vinegar solutions also work well when washing by hand. Taking the time to properly clean exterior wood makes a noticeable difference in the quality and durability of the new paint job.

Should you clean exterior wood before painting?

Yes, it is highly recommended to clean exterior wood thoroughly before painting or re-painting. Washing removes surface contaminants that can cause adhesion problems with the new paint coat. Some key reasons to clean exterior wood first include:

• Removes dirt, mildew, mold, and algae
• Eliminates chalky paint residue from previously painted surfaces
• Gets rid of sap, pollen, and environmental deposits
• Improves paint adhesion and longevity
• Allows stains and primers to penetrate better
• Prevents peeling or chipping of the new paint

Painting over unclean wood risks poor paint bond, peeling, cracking, or early fading of the finish. Taking the time to wash exterior surfaces thoroughly prevents many common paint failures.

How should you clean exterior wood before painting?

There are several effective methods for washing exterior wood prior to painting:

Pressure Washing

Pressure washing with a commercial power washer is the most thorough way to clean exterior wood. The high-pressure water can remove loose paint, deep dirt, mildew stains, and other stubborn surface contaminants. Pressure washing works well on siding, decks, fences, and larger exterior wood surfaces.

Detergents & Cleansers

Heavy duty detergents, degreasers, or commercial cleansers designed for wood preparation can help break down and remove dirt, grease, mold, and mildew from exterior wood surfaces. Apply the detergent solution with a stiff scrub brush and then rinse thoroughly with a garden hose.

Bleach Solutions

A dilute bleach solution helps kill mold, mildew, and algae growing on exterior wood. Mix 1 part bleach to 3 parts water and scrub onto affected areas. Let the solution sit briefly before rinsing off thoroughly. Bleach may lighten some wood surfaces.

Vinegar Wash

For mildly dirty exterior wood, a vinegar solution helps remove grime and freshen the surface for painting. Mix 1 cup white vinegar with 1 gallon of warm water. Scrub onto the wood with a soft brush and rinse off. Vinegar is a mild acid that cuts through dirt and chalky residue.

Sanding

Lightly sanding very weathered or previously painted exterior wood helps scuff up the surface and allows new paint to adhere better. Use 40-60 grit sandpaper and avoid sanding down to bare wood.

Cleaners containing oxalic acid, like Deck Brightener, also help prep and restore gray, weathered exterior wood prior to re-staining or painting.

Tips for Washing Exterior Wood

Follow these tips for effective cleaning and prep prior to painting:

• Pick an overcast, dry day to allow the wood to dry completely after washing.
• Wet the surface first, then apply cleanser solution starting from bottom up.
• Let cleansers sit briefly before scrubbing with a stiff nylon brush.
• Rinse thoroughly with a pressure nozzle to remove all residue.
• Allow 2-3 days of dry weather for the wood to dry fully before priming or painting.
• Use latex primer and paint formulated for exterior wood surfaces.
• Caulk gaps after cleaning – allow caulk to cure fully before painting.
• Lightly sand very weathered gray wood to help new paint adhere.
• Wear gloves, eye protection, and use caution when pressure washing.

What kind of cleaner should be used?

The best cleaners for prepping exterior wood include:

Cleaner	Description
Pressure washer	Uses high-pressure water jet to remove loose paint, deep dirt, etc.
Exterior wood cleaner	Detergent made for cleaning and brightening exterior wood.
Bleach solution	Kills mold, mildew, algae. 1 part bleach + 3 parts water.
Vinegar solution	Cuts through dirt and chalky residue. 1 cup vinegar + 1 gallon water.
Deck brightener	Oxalic acid cleaner that restores gray, weathered wood.

Always test cleaners first on a small, inconspicuous area to ensure they don’t damage or discolor the wood surface. Read product instructions carefully.

How should you get into small cracks and crevices?

To thoroughly clean small cracks, crevices, and detailed areas of exterior wood:

• Use a small stiff nylon brush to scrub cleaner into intricate areas.
• Rinse cracks thoroughly by spraying water at close range.
• Use a toothbrush to get detergent into very small cracks and holes.
• Allow extra drying time for intricate areas and vertical surfaces.
• For deep cracks wider than 1/4″, fill with exterior wood filler after cleaning and before priming.
• Use a compressed air blower to force water out of cracks after washing.
• Inspect intricate areas of siding or woodwork up close to ensure proper cleaning.

Thoroughly cleaning detailed areas prevents localized peeling or chipping of the new paint finish.

How long should you let exterior wood dry before painting?

It’s important to allow exterior wood to dry fully before applying primer or paint. Recommended drying times after washing:

• 2-3 days of dry weather if possible.
• At least 24 hours minimum.
• 48-72 hours for woods with high oil content like cedar and redwood.
• Longer if constantly damp or humid climate.
• Check moisture content with a wood moisture meter.
• Wood should be under 15% moisture content before painting.
• Primer and paint don’t adhere well to damp or wet wood.

Allow extra drying time for intricate areas protected from sun or areas behind shrubs/trees. Using fans or dehumidifiers can help speed dry time after washing exterior wood surfaces.

Does newly installed wood need to be cleaned before painting?

New exterior wood also requires prep and cleaning before painting or staining for best results:

• Allow 2-3 weeks for new wood to dry out thoroughly after installation.
• Check moisture content – should be below 15%.
• Clean to remove handling marks, grease, and surface debris.
• Sand smooth using 80-120 grit sandpaper.
• Brush off dust and wipe clean.
• Apply wood primer within 2 weeks of installation.
• Read manufacturer’s prep guidelines for factory treated woods.

Proper cleaning and prep helps the paint better penetrate and bond with the bare wood surface on new exterior installations.

Does exterior wood need to be sanded before painting?

Lightly sanding exterior wood can better prep very weathered gray surfaces for new paint:

• Use 40-60 grit sandpaper for light sanding.
• Avoid sanding down to bare wood.
• Sand just enough to rough up and degloss the surface.
• Sand after cleaning wood for best results.
• Brush off all dust after sanding before priming.
• Apply latex wood primer shortly after sanding.
• Use caution when sanding wood with lead-based paint.

However, sanding is not mandatory if the wood is cleaned thoroughly prior to painting. Proper power washing or cleaning with detergents can often prep weathered wood sufficiently without sanding.

Can you use TSP to clean exterior wood?

Trisodium phosphate (TSP) is a heavy-duty cleaner that works well on exterior wood prior to painting. Advantages of using TSP include:

• Removes grease, oil, dirt, mildew, chalky residue
• Degreases and brightens wood surfaces
• Improves paint adhesion
• Low-cost and widely available

When using TSP powder, mix approx. 1/3 cup per gallon of warm water. Test on a small area first, as it may discolor some woods. Rinse thoroughly after cleaning. Wear protective gloves when using TSP.

Does exterior wood have to be completely dry before priming?

Yes, exterior wood must be completely dry throughout before applying oil or latex primer. Moisture content should be below 15% for proper primer adhesion. Wet or damp wood will cause bubbling, cracking, peeling, and other paint failures if not fully dry before priming and painting.

Allow 2-3 days dry time after cleaning exterior wood. Test readiness by sticking painter’s tape to the wood overnight – if condensation forms under it or the tape peels off easily, the wood needs more drying time. Use a moisture meter on thicker wood. The extra wait is worthwhile to ensure long-lasting paint bonds.

Conclusion

Cleaning exterior wood thoroughly before painting is a critical step for a long-lasting finish. Pressure washing provides deep cleaning for siding, decks, and fences. For individual boards or railings, detergent solutions, cleansers, or vinegar washes effectively remove dirt, residue, mold, and other contaminants by hand. Allow sufficient dry time before priming for best adhesion. Properly preparing bare or previously painted exterior wood provides the sound surface needed for the new paint to bond tightly and prevents premature peeling, cracking, or fading.