Pat. Its probably aluminum siding. which was pretty durable 30 years ago. the problem is the paint, if theres any left, will be very weak and thin. you might wind up having bare aluminum left when your done.

I have never softwashed aluminum siding. so maybe others will chime in. But I did have a similar problem when we roofed a house once. the rip crew got some tar marks on the siding. When we went to clean it off the paint literally disappeared. you could see the areas we cleaned were down to the aluminum.

I would be very cautious and at the least try a test spot.
You could always pass on my info for a new siding job, for a nice finders fee!

Thanks Erik I will pass on your info I didn't know you did siding as well. No finders fee is needed but thanks for the offer. I think I will stay away from it since she said it was painted...I don't need the headache.