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Spray the surface with the solution, then scrub it with a clean, lint-free cloth. A powerful degreaser will cut through dirt, grime, mold, and other problematic residue without affecting the temperamental zinc coating. Proceed in small sections until you’ve cleaned the entire surface.  Common household products like Comet, mineral spirits, and chlorine bleach can all be used to prepare galvanized steel for painting. If you’re attempting to paint siding panels, roof flashing, or other materials that have seen exposure to the elements, a thorough cleaning is necessary to eliminate any organic contaminants from the outer surface. Once you’ve finished cleaning the steel, let it rest until all traces of the degreaser have evaporated. That way, you won’t have to worry about the solution interfering with the action of the vinegar, which you’ll be using to rough up the smooth steel. If possible, do your prepping and painting indoors, or on a clear day when there’s little moisture in the environment. When working with an item that’s seen some wear, you may noticed a chalky or powdery film on parts of the surface. This can easily be buffed out with a high-grit sandpaper (120-grit or finer is preferable) and a little patience. Sand the steel using gentle circular motions until the exterior takes on a uniform appearance.  Afterwards, wipe the surface with a cloth dampened with warm water to remove any lingering dust. This chalky substance is commonly known as “white rust.” It forms when the thin layer of zinc coating the steel begins to break down as a result of age or exposure to the elements. Wet a clean, dry cloth with distilled white vinegar and wring out the excess liquid. Go over the galvanized steel thoroughly, applying more vinegar as needed. To ensure an even paint job, it’s important that it come into contact with every part of the exterior.  The acid in the vinegar will gently etch the slick zinc-coated finish, giving it a rougher texture that will allow paint to stick better. If you happen to miss a spot, you may end up with splotchy, peeling paint. This will give it plenty of time to eat away at the galvanized surface. The longer it sits, the more pronounced the etching effect will be, and the better your paint will adhere. You might even let it work overnight if the timeframe for your project allows. If you’re short on time, wait until the surface is dry to the touch before moving on to priming and painting.

Summary:
Clean the steel with a chemical degreaser. Allow the surface to dry. Scour older galvanized steel lightly to remove white rust. Wipe down the steel with white vinegar. Leave the vinegar to act for 1-2 hours.