I have been working on painting a guitar for almost two years now. (Sorry it's taking so long Stefano).
First I was so afraid of messing it up that I just left it alone. I did some research at guitar shops to find out if painting it, or wood burning, or spray painting it would effect the sound. The answer was always yes. So I decided on wood burning. Did my sketch a dozen times, used chalk to sketch it on the guitar and held my breath as I went in to do the deed. A few hours later, suffering from the fumes of burnt polyurethane and only two leg outlines in I gave up. My next attempt I decided to go ahead and paint the darn thing. Sanded a little off the top, wiped off the dust, held my breath and started. And I hated the way the acrylic paint was looking. So I wandered around my house and had a wonderful, potentially foolish idea came to me... Nail polish. I started painting in the design tiny stroke by stroke and it turned out beautiful! The key to working with nail polish is patience. each layer has to dry for an hour before the next one can go on. So it's taken forever to do this painting, but it's totally been worth it. Now All I have to do is take pictures and mail it to NYC. I would go myself, but I just can't spare a whole day to deliver it myself.