I just painted a guitar with products from my local auto paint supplier with great results - I will post pictures and a step by step after it's all rubbed out and assembled. It the mean time here are a few thoughts that may be helpful:
Find a supplier with helpful salespeople that will spend some time helping you choose the right product, and explaining how to use them. The guy I talked to had painted guitars before.
Paint products that are available in one region may not be available in another, because of local environmental laws, so it is tough to recommend specific products. Im in an urban CA city, and the VOC laws are pretty tough here. Different (more volatile) products are available just by going out of town.
A good (expensive) pro quality clear coat will spray better, dry quicker, rub out better, look better, and be more durable than a cheaper alternative (like a spray can or a bargain clear coat)
When using auto body primer to level the wood, seal the wood first to prevent the primer from soaking in (I used conversion varnish)
Spray in an area that is as free as possible from dust, and curious passers by as possible. (The booth I used was clean, but a shop employee walked into the booth the check out the guitar, and dropped some of his cigarette ash on the still wet clear coat - GRRRR!)
Think safety - Breathing the fumes can case injury or death, so get a good canister mask and use a properly ventilated booth. You don't want to get this stuff on your skin - particularly the catalyzer.
Some people will have you believe that you cannot buy automotive paint in small quantities - this is not the case with the supplier I bought from. They will mix your color to order on the spot in pints - just look through the car color catalogs and pick your color!
I ended up using acrylic paints - a metallic color coat, and a 2-part (conversion) clear coat. The result is stunning, but it was pricey - about $150 for a pint of color, and a quart of clear and the reducer and catalyzer that are required).