These miniature birdhouses look to be ripe for painting. They are a Dollar Store item for $2.00 each. I can see having fun with these!
Below, the experimentation continued today with a variety of substances to see what will stick to a hard vanished surface, to wit, the front and back of a guitar.
So far, fabric paint is a non-starter as is acrylic. Even sanding first did not work. I tried a satin finish over both the fabric paint and the acrylic and you can scrape it off with your finger nail.
The only thing that is working so far is the permanent metallic pens, the copper colour to the left and lower right, the circle, actually. This stuff does not come off. So permanent Magic Marker will probably work. Not a great selection of colours so might not actually be a good plan.
This makes me wonder if oil pastels with a hard gloss covering might work. If the guitar is for the purpose of art and not being used, no doubt, this will work. However, if being handled, and worse, played, it's difficult to know what will endure.