How would you handle aluminum siding that has been painted a few times and now has a chalky quality that rubs off when touched?
Any surface that has a chalky feel should be cleaned to remove the chalk before painting. The easiest and most effective way to do this is by pressure washing and using a chemical injection system attached for the detergent. Be sure to allow the siding to dry a day or so, to allow the water that is driven up behind the siding to work its way out before painting.