Painting guitars can be a bad idea, as normal paints can kill the sound of the instrument. But I was inspired by the artwork of Peter Cree, a guitar builder who uses colored lacquer and shellac to "paint" full-on artwork on acoustic guitars. He claims that his hand-painted finishing method produces a finish that is 2-4 microns thick - better than many commercial guitars. I couldn't find details of Peter's method, and I have no idea how to measure microns on my finish, but I decided to try and find some way to do something similar.
Before I get started, I want to say that I am NOT an expert luthier (stringed instrument maker). I tried my best to do my own research and follow the plans for my guitar design, but only time will tell if I was successful. If you choose to make your own guitar, please do your own research using professional guides. There are several amateur luthier communities online, as well as good books. I personally used How to build your own Acoustic Guitar - by Jonathan Kinkead, found at my local library.