The process for removing rust stains from asphalt shingles is relatively simple, but it will take some time, energy and care in order to clean your roof without mishap. Select a day that is clear but not too sunny, and get ready to work. Ultimately you will need to identify the source of the rust stains in order to resolve the issue permanently, but the actual rust removal process can be completed in the space of several hours.
Clean off your roof. This will help you identify the source of the rust stains (usually metal nails holding the shingles in place) and also remove any additional dirt and grime that makes rust removal more difficult. First, sweep off the roof so that it is clear of leaves, branches and other debris. Then, use the garden hose to rinse everything down to get rid of dirt, mud and buildup. If your roof is especially dirty, you can scrub the shingles with the push broom while you are hosing it down.
Treat the rust stains with vinegar. Put the vinegar in the pump sprayer, and put on your safety goggles to keep the spray from blowing into your eyes. Spray the roof with the vinegar. Saturate the shingles--especially those with rust stains. Before the vinegar dries but after it sits on the rust for a little bit (about five to 10 minutes on an overcast day), scrub it with the push broom or the scrub brush. Work your way across the roof from left to right and top to bottom. In many cases, this will remove the rust stains completely.
Tackle tough rust stains with oxalic acid. Mix the acid, which comes in powdered form, and water in the pump sprayer. Put on your gloves and wear your goggles again. Since every manufacturer is slightly different, refer to the instructions on the package for specific diluting instructions. Spray the acid on the shingles and leave it there for no more than five minutes. You can scrub it after the five minutes; then rinse the area down with lots and lots of water from the hose.
Rinse your roof. In order to remove all cleaning agent residues, you need to hose down your roof. You may even want to re-scrub the roof as well using the push broom. At this point, your rust stains should be gone and your roof should be good as new.