When it’s time to clean your wood deck in preparation for refinishing, a chemical deck brightener applied by hand-scrubbing may not do the job of deep cleaning, but a pressure washer will.

Tip

A pressure washer has many uses around the home, but if you prefer not to own one, you can always rent one from a home improvement center or tool rental outlet. The typical cost of a rental for one day is $40 to $100.

A pressure washer is a straightforward machine that can clean very well with only water. However, it can ruin your deck if it’s not used properly. This is a tool that is powerful enough to etch brick, but in the right hands, it can be gentle enough to wash siding. If you use it with too much jet pressure or place the tip too close to the surface of your deck, you can deeply etch the wood and cause the fibers to splinter. Take some time to become familiar with the machine before you unleash that cleaning power on your deck.

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What You’ll Need

Equipment / Tools

• Pressure washer with a fan tip or rotating tip
• Sheets of plastic (optional)
• Random orbit sander (optional)

Materials

• Deck stripper/brightener (optional)
• Sandpaper (optional)

Instructions

1. Select a Tip and Pressure SettingChoose the appropriate pressure setting and spray tip for your application:Use the lowest possible pressure that is still effective. Pressure for soft wood (like cedar or pine) should be at about 500 to 600 pounds per square inch (psi); harder woods may tolerate more pressure, but you should not go higher than 1,200 psi.Use a fan tip with a 40- to 60-degree spread. A rotating tip is also suitable if you use it carefully. 
2. Test the Pressure Washer SettingsUsing a pressure washer takes a little practice. Test your pressure and its effectiveness in an inconspicuous area, such as a stair tread or corner. If you find it necessary to replace a piece of wood because you etched it, a stair tread is easier to replace than a surface deck board.Start with the pressure at 500 to 600 psi. Increase the pressure incrementally until you find the right setting for proper cleaning.
3. Power Wash the DeckEngage the trigger while holding the wand tip a couple of feet away from the deck surface, then lower the wand closer as needed. Try not to get closer than six inches or you could damage the wood.Clean the deck with a sweeping motion, and avoid the tendency to pivot with your arm, which will result in ​an inconsistent distance between the spray tip and the deck surface. Try to maintain a consistent distance by moving your arm laterally back and forth.Clean the deck boards from the house outward. Work with the grain by feathering the spray lengthwise, parallel to the deck boards, and overlapping each area slightly. The goal is to achieve even cleaning with no “hot spots” or visible cleaning edges.
4. Apply a Chemical Stripper/Brightener (Optional)If the wood is badly darkened or stained with mildew, washing with a deck-cleaning solution that includes sodium hydroxide can help brighten it. Look for a product designed for brightening deck wood and for use in a pressure washer, and make sure your pressure washer allows for the intake of cleaning solutions.Mix up the solution as recommended by the manufacturer. Apply the stripper solution in the same manner as when pressure washing with plain water, then rinse by washing again with plain water. Stripper/brightener products tend to roughen up the wood fibers, so you will generally need to sand the deck completely before re-staining or sealing the wood.

Sanding After Washing

Ideally, power washing a wood deck will not ruin the wood and etch or erode the softwood fibers. However, when wood gets wet, the fibers may become raised, creating a rough surface. This is a particular problem on handrails, where the raised fibers can lead to splinters and require some sanding.

You have a couple of options for remedying this problem. If you did a great job of power washing with minimal damage and you just want to clean the deck, you may be able to get away with a spot-sanding here and there. But if you are going to refinish (re-stain or reseal) the deck, you will need to sand the entire deck to create smooth boards with little to no splintering. Sanding also opens up the wood’s pores so the wood soaks up the sealer or stain consistently. Make sure the wood is dry before sanding.

When sanding, do not use very fine sandpaper, since this can clog the pores of the wood and prevent the stain/sealer from readily soaking in. For the handrail, use no finer than 100-grit sandpaper. For the deck surface itself, use 60- to 80-grit sandpaper. The best sander to use for a wood deck is a random orbit sander with a five-inch sanding pad.