It's a bit of manual work, but it's not hard. If you use the basic painting and prep procedures, whether you use a gun or rattle can is actually of little concern (or consequence). In other words, with sound methodology, and time and care, you can get simply amazing, and surprisingly professional results...out of a rattle can.
I am a bass guitarist, and on a bass forum I frequent, people are always posting super-professional jobs they've done on guitars using a rattle can, using paint from here:
http://www.reranch.com/
This site sells paint. What does this have to do with automotive paint?
Guitars are actually often finished in car paint, some very famous traditional vintage guitar colors actually being nothing more than a GM paint number.
I'll see if I can come up with some links on how-to's, but I wouldn't be doing much more than using google myself. I'm sure there's a million out there, some in video form on YouTube, if you don't even care to have to read it.