sheet where 4X8 0.04 tickness. if i had travel to vancouver, they have 8.5 feet wide 0.04 aluminum in coil...so you can buy as long as you want. it s great for bigger size but very ackward to handle.
For the paint, i used 220 Grit on a rotating sander. scuff very marginally, clean up very well with water/soap, then acetone and immediately prime with zinc chromate. i found this way to be very good to hold the paint and not chip. the key is to prime within minutes of sanding so no oxidation can occure.
i used Rustolum rust paint (enamel oil base) . that is the cheap kind you can find in any hardware store. in canada, all hardware store also have there own home brand rust paint that is made my rustoleum. the good thing is..it s made in base paint so you can get any custom color you want for about 10$ a liter or 26-30$ a gallon. this paint finished VERY glossy and is extremely resistant. it look like a professional car paint job from 5 feet or further away. in term of longivity.... i finished my airstream a few years ago in bright red and so far... it look like the day i painted it. (applied with roller)
airstream argosy rustoleum pained on side of the road!