Madd, if you wish to paint galvanized metal, before using a regular primer you must first use a self etching primer. Eg: Martin seniors "Trio Prime" is an excellent product. I have had excellent results with this and have never had a problem with flaking paint etc. Caution is addvised with this product in that you want to have good ventalation and personal safty equipment. read the product data sheet.
This works well for full coverage and for spot repairs. I hope that you find this info helpful...Martin