I agree that proper primer is a good idea in most cases. I paint my own bodywork and use PPG primer and urethane with hardner. But for my wheels, I took the easy route. I washed the Panasports with Simple Green, taped up the silver rims and shot the gold paint out of a can. It took about a half hour and the wheels haven't chipped thru 4 races.
The best thing is the paint cost me $1.50.
Wal-Mart has 3 different shades of gold metalic that comes in a small size spray can for $1.50.
The can will do one coat on 4 wheels. If they chip, I'll just spray them again.
Go to a NAPA(or similar) and get a self etching primer. This will adhere to the alloy surface of previous paint finish and is very tough. Your new paint will have a great bond. Adding a flex agent is correct if you are using a base/clear system. But finish with a good urethane clear coat and you should be all set!