removing rust and primer to paint

I was wanting to know if anyone could HELP me please....I has a 85 Camaro I bought for $500.00 for a project car and it has rust all over. I has tried sanding and primering the roof and rust keeps popping up with in a few day. I knonw I'm doing something wrong and would like to know if someone could help me...by tell me how to go about removing the rust and primering. I would love a step by step or anything. If you know of a good website that could help beside this one of course hehe...

it re-apears because blive it or not the moisture from the evening is getting to the bare metal, you have to cover it and store it in a garage until your ready to paint.

take it down to the metal.. make sure you get ALL the rust from around the area... Spray it with a bare metal primer (they usually contain zinc, or zinc oxide) wet sand it, prime it again, wet sand it, then paint it in 3 or 4 light coats instead of one heavy one.. Make sure, before spraying any primer or paint onto the car, you wipe it with acetone, or similar to remove any grease from your fingers, moisture, and sanding residue and such... Then the rust should stay away for a while...