Like dirk says, you'll want to use a primer with acryls. Unlike enamels or lacquers, acrylics don't 'bite' to the plastic, rather they sit on top. They're easily scratched off. You want an inbetween layer that bites down, which your acrylics bond to, and that's a primer. I've heard Tamiya makes a primer, I've never used it personally. I've used Testors from a rattle can, I really didn't like it too much. I've found Vallejo's light grey primer works well, I'm told their white doesn't work too well. I'm told Mr. Surfacer is a really nice product too, but I've not used it. Then I'm sure there are generic art store primers, automotive primers, I can't speak for those, but I'd use something meant for styrene.