Why Do You Need to Wash Your House Anyway?
Washing down your house exterior is just one element of preparing to paint your house.
You would not expect your new paint to attempt to stick to crumbly, flaky, failing paint. In the same sense, when a fresh coat of paint tries to stick to dirty siding, it too will fail. The problem is that failure does not happen right away.
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.
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.
Manual Cleaning is Preferred
Why no pressure washer? If you use enough 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.
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: Tri-sodium 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: it gets heavy after awhile.
Garden hose: An ordinary hose and nozzle.
Wire brush: Helps to brush away stuck-on mud and remnants of long-dead wasps' nests.
How to Manually Clean Your Siding
Now that you have your tools together, pick a warm, sunny day and begin cleaning. Remember to work from the top-down.
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: dried mud, spiderwebs, bird nests, vacated wasp nests, etc.
Gently spray one section of the siding with the garden hose. Caution: 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.
Think your siding is already clean? Try mixing up fresh TSP-water solution again and re-scrubbing it. You will be surprised at how dirty this second batch of water is.
Move on to the next section below that one.
When all sections are done, spray the entire house side once again.
Let dry by itself at least 24-48 hours before painting.