Primer is porous. Hence the reason why it's primer and not topcoat. Constand exposure to UV breaks it down, constant exposure to water will work it's way through. For corrosion protection of the metal under the primer your best bet is to blow something meant as a topcoat over top of it. Something other than spray bomb.
Never ceases to amaze me watching guys put a car in primer, no matter what the primer, drive it for a few years, blow paint over it and then scratch their heads over the rust bubbles popping out less than a year later.
