I'd say put a coat of primer on it just in case no matter what you use. I've had luck with spray paint but at the same time I also has a bad time with spray paint thanks to some odd reaction with the flat clearcoat I used. I've never tried anything but spray paint as I hate brushing paint on.
there was a huge discussion on this topic on sibertron.com. I myself use acrylics with a sealer afterwards. some others have gotten good results with enamels. its all in the user I guess.