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Before pressure washing, you can spray your deck lightly with a deck cleaning solution made of equal parts oxalic acid and water. Oxalic acid has a brightening effect that will lighten mold stains and return the deck to its original freshly-sawn look. Allow the solution to sit for about 20 minutes before pressure washing so it has time to take effect.

Yes, you must thoroughly clean your deck before staining. If you don't, dirt, debris, grease, and mildew can get trapped between the wood and the new stain. This not only looks unsightly but also prevents the stain from penetrating the wood properly, reducing its ability to protect the deck from weather and UV rays. As a result, the stain won't last as long or look as good. Any existing mold or mildew left untreated can continue to grow beneath the new stain, potentially leading to wood rot and structural damage. Always wait at least 48 hours after cleaning for the wood to dry completely before applying stain.

Much like wood siding, it can be dangerous to use a pressure washer on composite siding. While normally durable, composite siding isn’t made to stand up to higher pressure settings. It also makes it very easy to damage the paint or finish on the siding. Some professional power washing companies may take the job on, but we recommend avoiding it, especially as a DIY project with a rented pressure washer.

Cedar shake siding requires regular cleaning, or else it will grow mold and algae. However, a pressure washer can easily damage your cedar shakes if the water pressure is too high. We recommend cleaning with a garden hose instead. Review our guide to cedar siding maintenance if you’ve just finished your installation.