I would start at probly a wet sand paper 220 work your way up to a 600 or so. maybe use a razor blade to take the stickers off, prime it, and sand the primer with like 600 or 1000 grit. I would'nt use rustoleum
paint, and the gas will eat at it if you spill a little on the tank, and make it turn brown. I would go with some engine paint or such from an automotive store (napa, advanced auto, parts city, etc) That paint seems to hold up a lot better. If it were me, I would rather paint it with a pneumatic air gun, but not everyone has the setup. Remember, prep is the key to make the final result look the way you want it.
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes