If mold or mildew is the culprit spraying with a bleach solution will remove it.
How to remove algae from aluminum siding.
It s not uncommon to have a mildew problem on aluminum siding depending on where you live.
Mildew and algae on vinyl and wood use a solution made of oxygen bleach not chlorine bleach mixed with water.
For those tough spots use a gentle.
Spray this mixture on remaining spots.
Stains on wood cleanse with mild detergent mixed with water.
Choose a cleaning solution that is appropriate for the type of growth you have on your siding.
To 1 quart and use it in the same power washer as step 1.
To clean aluminum siding having mildew use a solution of one part bleach and four parts water.
Water stains remove with a mild detergent and water.