As Jon politely corrected your looking for a double action brush. This means you have a double action ... lol . Ok its to do with the trigger, with a double action you push it down for air, you then pull back for paint. With a single action I think you just pull it back and you get both? Not quite sure on that, but whatever it does, it's not want you want for this.
Gravity feed I think the advantage is gravity ... lol The paint flows into the nozzle easier...
Paint is readily available from such places as RCS Graphic Worx (click on the Jons banner in his signature) . Water based stuff like Createx and Jon's range of water based is probably a good place to start. You can strip the paint off, however it'll get messy and it's not worth it in the end. Your best off just practising on Lexan sheets or maybe Jon's inexpensive body shell practise sets.
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