Long term is the problem Rich.
To do it properly, you`ll need to make sure it is properly etched. We use an all in one etch primer/ hi build primer. The frame will also need a light rub/sand just for extra safety. Steell wool (scotchbrite pads) will do.
On a sign, you`d get away with using a standard polyurothane paint (single pack). 2 Pack is a little more expensive and requires a slighty better tecnique at applying and dries much harder and a little faster. It`s mainly based at the automotive industry whereas single pack paint covers most other areas.
Give Scotpaints a call in Chapelhall (01236 767666) and ask them for the price difference. You might also wanna explain to them what psi your compressor is putting out etc. They`ll advise what paint to use to suit your setup.