Galvanised metal is iron or steel protected with a zinc coating. The zinc coating -- or galvanisation -- prevents the metal from staining and rusting. Galvanised metal is used for a variety of purposes, but mostly for roofing. Before painting galvanised metal, you must cover the zinc coating with primer. Priming galvanised metal is necessary before painting. Primer prevents the paint from peeling and flaking away from the galvanising coat after it dries. There are primers specifically designed for galvanised metal.
Prepare your work area. You need a non-humid work area that is at least 21.1 degrees Celsius and ventilated. Humidity will cause the primer to bubble and cold temperatures slow the time the primer requires to dry. Cover the floor with plastic at least five feet longer and wider than the galvanised object you are priming. If not, the pressure sprayer will fade the floor with primer.
Wipe the galvanised object clean with a wet rag. Dust, mildew and precipitation prevent the primer from adhering to the galvanisation. Allow the object to dry while you check the fluids on the pressure sprayer and fill it with primer. Put on your safety equipment and test spray a piece of cardboard. This allows you to get a sense of how close you must stand to the object when spraying without the primer bleeding down it. You want a nice even coat, but you do not want puddles or streaks.
Spray the galvanised object, using even strokes. Do not stay in one place too long. If you under spray an area, you can always return with another pass. However, if you over spray the object, you will be resigned to wiping it clean, allowing it to dry and starting again. You know your layer is thick enough when you can no longer see the metallic colour of the metal. The entire object should be a dull grey.
Allow the primer to cure for 12 hours. If the object needs a second coat, be cautious about spraying it from too close a distance or in one area for too long. Removing puddles and wiping off bleeding is difficult after the first coat is applied. Allow the primer to cure for 24 hours, then your galvanised object is ready to paint.