if you dont use the correct primer it'll peel or flake after a few months mate. Even more so if you use a carwash or power washer!
if you go to your local body paint shop and ask their advice they should be able to russle up some clear base coat stuff (cant remem its name) that you spray on first... this effectively gives a smooth finish that the paint can grip to better.
trust...its worth the investment.
95% prep
5% painting!