Anyone can throw paint over galvanised steel, but without proper preparation any attempts to do so will be an exercise in failure. From boats to doors and even patio furniture, knowing how to paint galvanised steel is a necessary skill for virtually any crafty person. Galvanised steel is treated to resist staining by adding a zinc compound to steel in extremely high temperatures. In order to properly prepare galvanised steel for painting, it is important to use proper measure to partially strip this zinc coating down and create a rougher surface that the paint will be able to adhere to.
Wash the steel with a water-based alkaline cleanser solution. It is important that the cleanser is water-based as this is the only way to ensure that the cleanser does not leave behind any residue. The alkaline solution should have a pH rating between 11 and 12. For newly galvanised steel surfaces (2 days or less) this step should be all the prep work necessary before painting.
Rub down the steel with the sandpaper using a gentle, even stroke. This will remove most of the galvanised finish and provide a better surface for the paint to adhere to.
Wipe down the steel with a clean cloth to remove any dust or loose debris.
Prime the surface with a paint that will serve as a "tie coat" between the galvanised steel and the top coat. Because the chemical make-up of each paint is different, consult with a paint expert of the paint manufacturer in order to determine the best paint to apply to steel. See the "Resources" section for a good reference for metallic primers.
Paint the topcoat over the primer surface. Again, each paint is made differently so consult with an expert before applying the paint. Paint a small test area first to make sure the pain adheres properly. Allow the paint ample time to cure (curing on metal can take up to two weeks). Generally acrylic latex paint works well.
A solution of equal parts ammonia and warm water can also be used in the first step in place of an alkaline solution
These steps will not work on chemically-treated galvanised steel