I have tried to reclaim MANY projects that had been coated with rattle can primer over months, or even years. Here's the problem: Primer is porous, and if it gets rained on, there's a good chance a thin layer of rust will develop down at the metal surface. Even if the top surface is sanded and coated with a good epoxy-based primer, the rust will be there to erupt like a volcano at some time in the future. Most professional painters I've known will simply refuse to paint a vehicle that's had primed areas exposed to rain, unless they're allowed to sand all the way back to bare metal and reprime those spots. Putting a color coat over a possible layer of rust would be like applying a Band-Aid over a boil getting ready to come to a head.
Jerry
