The thing with rattle can paint is that it's not hardened, so it sands away very very quickly. I'd do a couple heavier coats of each color, giving at least overnight for each color to dry before spraying the next. Sand as you want, when you're happy, get a can of SprayMax 2K clear. It's a 2-part rattle can, you "activate" it, which releases a vial of hardener into the can. Then clear over your job to prevent nicks and scratches. Gasoline will also readily take off non-hardened paint, something to consider for a gas tank.