Trouble with undercoat

I am having some trouble painting my armor. It's made of Sintra and someone told me you dont need to prime or sand it before you paint it, he also suggested just using regular spray paint for the silver undercoat. So I sprayed on some silver Krylon paint, but even after it dried the paint just kinda rubs off into a sorta dust whenever I handle it. Should I sand the paint off and try again, but prime/sand it first? And should I still use that spray paint? (I havn't gotten around to buying an airbrush kit yet.)

It is best to use a primer, especially when painting a plastic. Give it a light sanding to make sure the surface is ready, then prime. What you are describing sounds like a defective paint. Is the paint coming out fairly dry?

Silver paint has an oxide in it that will rub off, any silver/aluminum/chrome paint will come off on your hand if you dont let it dry for awhile, some material has different drying times, but if you rub hard enough the silver will come off of any material, its just the way it is.

Like xaoslord said its best to use a primer.