I have bought 4 sets of mirrors for my truck, all of them "stainless". They have all started rusting within a month! The cheap stainless'es rust like mad. I rather think they have trace amounts of chromium, etc and so some marketing liar will call them stainless. Realllll annoying.

You may have gotten a sheet of this psuedo stainless. In other words you need to get actual ASTM rated stainless for your projects.