How to Clean Your Siding Before Painting

Unlike interior painting–which allows for some leeway with cleaning–you must clean your house's siding before painting. There's just no way around it. 

As an experiment, rub a clean white cloth across the unwashed siding. The cloth will likely come up dark-gray with a layer of fine soot. Even after pressure washing, you may still find that a thin layer of dirt remains on the siding.

Paint applied to dirty siding will adhere in the short term. But over the years, your house will experience paint failure faster than if it were applied to clean siding. So, washing the siding is always recommended.

Manual Washing vs. Pressure Washing

You can either clean the siding by hand for a slow but deep-clean job or you can clean it with a power washer for a faster but less-thorough job.

Manually Wash Siding

Using brushes, a bucket, a hose, and a cleaning agent such as TSP, you scrub every square foot of the siding by hand.

Pros

Manual washing is considered the best way to clean your siding since hand-rubbing with a brush can scour off dirt better than water pressure alone. Also, this gives you a chance to inspect and repair the siding.

Manual cleaning is by far cheaper than pressure washing (if you do not already have a pressure washer). These are the supplies you'll need:

• TSP: Trisodium phosphate is one of those great little secrets hiding in your hardware store. It is a powder (though can be found in liquid form, too) and is cheap. Mix half a cup with two gallons of warm water to make an all-purpose cleaning solution that leaves no film. Get a big five-pound box.
• Mildewcide: If your house has mildew, you can speed up the cleaning process with a dedicated mildewcide.
• Hand brush: A stiff-bristle hand brush roughly nine feet long.
• Long brush: You need to be able to reach an extra three or four feet. Your hand brush might have a place to screw in a broom handle. If so, you can use this. To avoid continually dipping the brush into water, you can use a siding and eaves brush that runs water through the handle up to the brush. One downside is that it gets heavy after a while.
• Garden hose: An ordinary hose and nozzle.
• Wire brush: Helps to brush away stuck-on mud and remnants of long-dead wasps' nests.

Cons

Washing down a house exterior by hand is difficult manual labor and is potentially dangerous, as it involves climbing ladders. Also, if you're not expedient enough about washing down the siding, you may find that the first sections you cleaned have gotten dirty again.

Pressure Wash Siding

With this method, you purchase, borrow, or rent a power washer and spray the house down thoroughly.

Pros

This is a recommended method when you want the absolute best adhesion surface for your paint. Less manual labor is involved. More coverage in less time is possible.

Also, this is a good method when time is of the essence. Painting season may be waning and you need to paint sooner rather than later.

Cons

With pressure washing, there is the potential of siding damage, water injected under siding, or dirt remaining on house. There are ways to safely clean your house before painting using a pressure washer, including using lower psi pressure washers, widening the spray, and standing at least two feet back.

If you use too much force to blast away all of the dirt, you stand a good chance of etching your siding. Yet if you hold the nozzle farther back, it is not enough to remove all of the dirt–just the majority of it.