As an airbrush artist, I have painted quite a few bikes and Taz has given you great advice. You must prep and reclear the entire tank, not just the area you are airbrushing. If you are using water based paints for your airbrushing, I suggest sanding with no finer than 800 grit. You need a bit more tooth for the water based paints. I also like to go over the 800 grit sanding with a grey scotchbrite pad just to ensure a super smooth surface and to make sure I didn't miss any hard to reach areas.