One thing my prop-building boyfriend has drilled into my head is always ALWAYS put a couple of light primer coats down first on anything you paint. It makes the paint cover more smoothly, especially if you wet-sand each layer with a fine grit sandpaper. Then do several light layers of the paint, letting each layer dry at least 5-6 hours to cure completely (even if it looks dry to the touch soon after painting, it is still not completely dry), and don't paint in high humidity as the paint won't cure correctly and will always be slightly sticky. If you have to paint outdoors in humidity, do it very quickly (light layers) before the condensation sets in (especially if your piece just came out of an air conditioned house) and then quickly take it back inside to dry. Then, like Melonpool suggested, clear-coat the final layer. You wouldn't think taking all those extra steps would make that much of a difference, but it really does. The color primer you use depends on the color of the top coat. He just painted some vac-formed styrene white (which, ironically was white to being with, just not white enough) and I think he used that rust-colored primer under the white. Hope that helps.