Firstly, I'd say don't paint anything you value, just because it might go wrong! Secondly, with a guitar, it's a good idea not to paint the fretboard, as it effects the ease of sliding your fingers up and down; neither Johnny nor Chris have painted fretboards. Thirdly, I use acrylics, and I believe Coldplay do too. Acrylic varnish you can find online quite cheaply.
As for how I paint, I just paint a section in one colour, wait for it to dry, then do the next. Mix it together if I want that effect, simple enough. If you want any clear images, then paint the whole thing, then print a stencil, then paint through the stencil!
I'm going to paint my guitar, so I was looking for inspiration.. I just saw your post, and I really like your painted instruments! Especially the Viva-guitar!
Can you please explain to me how to you did that?
Thanks a lot, have a nice day!