I did a job today where I soft washed an old farmhouse with some fairly old aluminum siding. When I bid the job I rubbed a few spots on the different sides of the house to see how badly oxidized the paint was. I let the customer know that I couldn't guarantee that they wouldn't have some paitn loss in some areas though. They were fine with that since they felt anything was better than how dirty it was. They had also tried brushing off the algae and dirt themselves to no avail.
Do you have any other ways you "test" old aluminum siding on a job like this?
I have some pictures of the job here