You probably have seen it before, paint peeling off exterior doors, gutters or railings like potato chips. The surface below is a gray colored metal that might have a white powdery surface. This is galvanized metal and needs to be prepped and painted with the proper materials in order for this not to happen. It's not as hard as people think but certain rules need to be followed for a successful paint job.
STEP 1: Clean and Prep the Surface
The surface needs to be cleaned and prepped before any paint can be applied. New galvanized steel can have oils and chemicals that are applied by the manufacturer to protect the steel. These need to be removed and the steel needs to be dulled/etched for good adhesion. The easiest way to do this is use a product like Krud Kutter Metal Clean & Etch. It will clean and etch the surface at the same time. It is mixed with water and applied with a sprayer or sponge (make sure to wear rubber gloves and safety glasses). Let it sit on the surface for 10 minutes and then rinse with water. This will take care of the prep for new galvanized surfaces. If the galvanized steel is weathered and has a white powdery surface the steps are different. The white powder needs to be removed. This can be usually be achieved by sanding the surface. If it is really bad grinding may be necessary. After the powdery substance is removed it can be cleaned with a water based cleaner like Simple Green.
STEP 2: Painting
The most important rule to remember when painting galvanized steel is DO NOT USE OIL/ALKYD PAINT. Whether it is primer or paint oil based products will not stick to galvanized surfaces long term. An good acrylic or even epoxy paint will work. In most cases a primer is not even necessary. I would recommend an acrylic dtm product like Sherwin Williams DTM Acrylic.