A.Jon Tobey, a painting contractor in Monroe, Wash., responds: Most of the time, a pressure washing with detergent solution is adequate for removing chalking and dirt from aluminum siding. Kill any mildew that's present with a 3-to-1 water-bleach mixture before washing.

If there is some water-activated staining that won't wash off, lightly fog the siding (at the rate of about 3 gallons per house) with a quick-drying alkyd primer. If you're brushing, you can buy aerosol cans of the primer and spray the stains with a very light coat (one you can still see the stains through). In rare cases, there may be some bare...

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