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.
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.
- 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.
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.
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.
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.
Equipment / Tools
- Pressure washer
- Garden hose
- Siding detergent
How to Manually Clean Your Siding
Start With One Side
Choose one side of the house as your cleaning project of the day and stick with it. It helps to mentally cordon that side into half or thirds, as an entire side of the house is too much to clean at once.
With the siding dry, remove large pieces of debris with the wire brush, broom, and even a shop vacuum, possibly including dried mud, spiderwebs, bird nests, vacated wasp nests, and more.
Spray Siding With Hose
Gently spray one section of the siding with the garden hose.
Do not spray upward under siding or into air vents near the eaves which permit air circulation into the attic. In general, be aware of any place that may allow water to intrude under the siding or into the house. Also, look for dryer and bathroom vents and around poorly-fitting windows.
Scrub down the section with a prepared TSP-water solution.
Rinse off the section with clean water from the hose.
Scrub Siding Again
Mix up fresh TSP-water solution again and re-scrub it.
Move to Next Section
Move on to the next section below the previous section. Work from the top-down.
Rinse Entire House
When all sections are done, spray the entire house side once again.
Let Siding Dry
Let dry by itself at least 24-48 hours before painting.
How to Pressure Wash Your Siding Before Painting
Plug in the pressure washer. Attach the hose from the house to the pressure washer. Make sure that you have enough hose to reach the farthest part of the house exterior.
Hit the side of the house and especially the eaves to knock off large pieces of debris like mud, spiderwebs, and bird nests.
Spray Siding Down
With the sprayer on wide-spray, wash down the siding from the top-down.
Apply Cleaning Fluid
Switch the pressure washer to its soap setting. Spray down the siding from top-down completely with the detergent solution. Let it sit for about 15 minutes.
Switch the pressure washer back to its rinse setting. Rinse the siding from top-down. Completely remove all soap. Keep rinsing until the rinse water runs clear.