Before painting though, you'll want to make sure that your siding is free of dirt and debris. Painting over things like dirt, bird droppings, and other kinds of filth is a bad idea because if that filth comes loose, so does the paint that covers it, leaving an unprotected and unsightly spot on your home.
The best way to blast away the grime is with an electric pressure washer, but there is a right way and a wrong way to do so. While pressure washers can pack power capable of stripping the old paint, that's not what you want to do here. Use enough pressure to get your siding clean, but not so strong that you strip paint.
You want to limit the amount of water coming into contact with raw, untreated wood. Removing paint with a gas pressure washer will saturate the raw, untreated wood underneath. This can cause water damage and weaken your siding.
Start by spraying with clean water to knock off the larger chunks and globs of dirt and mossy build-up. The idea is to get rid of the big stuff so the detergent spray can penetrate to the filth beneath it.
Next, begin spraying a detergent mixture from the bottom-up, keeping the nozzle angled downward as much as possible to avoid forcing water up under any of your siding. Let the soap set long enough to loosen the dirt, but don't let it dry.
Finally, use another clean water spray to spray away the soap and dirt in a downward motion from top to bottom. Make sure to let your siding dry for a solid 48 hours before painting. You want to be sure you're not trapping any moisture under the paint.
If you can't get around to painting within a couple of weeks after pressure washing, it's a good idea to pressure wash the siding again with clean water only, then let it dry for another 48 hours prior to painting.
