If it is painted, you will not gain anything by priming over the paint. Make sure it is clean though.
Then I would use One Shot Sign Painters Enamel. It is an alkyd and is nice because it flows nicely and will not crawl. Paint stores may have it but if not your art supply store should have most colors or can get them in a day or so.
Another option if you plan on it being very large and it is intricate? Output it to vinyl and apply that to the surface almost like a large decal. Some sign shops have the capability as do places like Fast Signs and even larger Kinko's. The folks that do vinyl wraps for vehicles will certainly be able to handle the task.