Ah! That's simpler, but time consuming. A rough paint job will still get pushed past Mach. But it sounds like you want every ft/s you can get. The trick is surface preparation. If you can feel it, you'll see it when painted. Sanding starts at 220, prime, fill indents/divots/scratches/pinholes (bondo spot and glazing putty, the red stuff not the 2 part), sand ~320 or 400 grit now, repeat till smooth. When primed surface is as flat as you're willing to get it, paint. Follow the instructions to the letter as far as recoat times and environmental conditions. Spray cans or airbrush is up to your skill/resources. When fully cured, wetsand increasing grits 600, 1000, 2000. Some folks use a polishing compound at this stage.