Is an Aluminium or Plastic Paint Container Better for Air Painting?
Honestly I use a plastic cup and haven't had any problems, despite the fact that I shoot a 2-part LPU (what Boeing paints the Dreamliner with) and the solvent for the stuff is so caustic it's a paint stripper for latex and some aklyds. I expected it would have wrecked the cup, but it didn't.   

I am partial to a very well rated touch up gun I found on Amazon.  Despite being cheap Cheese, it's a bulletproof little thing and about $35.