Just use brake cleaner. It could very well be that an agent in the windex is causing a chemical reaction with the paint. The good news is that this can be fixed. Sand the entire caliper and use brake cleaner to clean it up. The brake cleaner *may* cause the paint to come off. The import part is to make sure that every part of the caliper is roughed up so the paint will stick. And you want to make sure it is clean for the same reason. Also a high temp laquer will also give you a smooth glossy finish. It's more $$$ and more time consuming, but it looks great! If you go this route make sure that the laquer will not react with the aluminum caliper.