Galvanised sheet metal has a zinc coating that protects it from rust. This layer also inhibits paint adhesion. If you slap a coat of paint over unprimed galvanised sheet metal, paint failure will inevitably occur. Unfortunately, ordinary base primers won't adhere to galvanised metal. Select a special self-etching primer, specifically formulated for galvanised surfaces. Apply the etching primer and finish coat in a particular way, or flaws may appear in the finish coat.
Use steel wool or a coarse plastic sponge to scrub the galvanised sheet metal with a degreasing cleanser. Rinse the sheet metal, using a pressure washer or wet rags. Allow two hours dry time.
Apply self-etching primer to a 3-foot by 3-foot portion of the galvanised sheet metal, using a roller. Smooth the wet etching primer to promote a flawless finish. Brush the sheet metal in a vertical motion, applying gentle pressure. Continue until all of the sheet metal is primed. Allow the sheet metal to dry for three hours.
Wash the self-etching primer from the roller/brush, using water.
Paint the galvanised sheet metal just as you primed it. Wait three hours for the final finish to dry.
Never use latex primer on galvanised sheet metal, or the finish will flake. Do not use a nylon brush to finish galvanised sheet metal, or flaws will appear in the finish coat. If you paint unprimed galvanised sheet metal, the finish will peel.