Really quick, if you paint something with the paint at the auto store I would strongly suggest also putting down a few coats of clear coat. What ever brand paint you end up getting, there should be a can of clear near by. You can then polish up the clear coat like you would your normal paint.
Also, I would suggest taking your time painting. Give one light coat of paint each night for a couple nights, and then a few clear coats then let it dry for a couple days.
A friend of mine and myself have built a few custom guitars and we always use auto paint and this is how we do it. If you take your time it will come out really good.