To your credit,.that painted aluminum looked to be in a very oxidized state,.and you really didn't do anything "wrong",...But,.it should have been evaluated as such and explained to the customer that the condition of the paint on the aluminum was bad,..and it wasn't gonna handle much of any type of cleaning around it. Could have been covered,.but I'd explain it as" needing" painted anyhow and I'd recommend having it prepped for paint. But,.those things need to be talked about before the work begins. Most everyone knows someone who paints,..and is nice to have a couple numbers on hand to fix this type of thing. If I were you I'd just go remove the rest of the oxidation and have it painted,.the customer may understand it was in bad shape and even though they weren't planning on painting,.they may just have it done at their cost,..or at least maybe do a percentage with you. Don't sweat it though,.not a big deal really.
Jeff