You can paint aluminum just fine, especially tunnels. I paint most of mine, prep is important but then use a good self etching primer for aluminum and and a true two component epoxy paint works great, most auto paints are to hard, so if you use a 2 stage auto paint get one that will flex without cracking for a sled. Lately I have been using POR's whitecoat/hardcoat epoxy paint and just tinting it to whatever color I want, my M1000 has been 2 seasons on the tunnel and it is still perfect, you can order all the stuff from POR 15 including the primer, but is nasty to shoot use a fresh air resperator.