Bare cast iron is nonporous and too slick to support long-lasting painted finishes. If you try to paint directly over the top of bare, untreated cast iron, rampant finish failure will result. The proper primer improves cast iron's adhesive qualities; however, the wrong kind will have no effect. Learn which primer is right for cast iron and which ones are wrong, or you may end up with a flaking finish.
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The wrong primers
Water-based latex and acrylic primers do not work on cast iron because the surface is too nonporous to absorb the primer adhesives. If you use either one of these primers on a cast iron surface, flaking will occur within months of application. Oil-based primers tend to provide slightly better results, but even these chip away after a while. For lasting results, avoid primers with these characteristics.
The right primer
Professionals abrade nonporous plastic and vinyl surfaces to enhance adhesion. Unfortunately, cast iron is far too hard and durable for this technique. Instead, condition the cast iron for better adhesion by applying a specific type of primer formulated with the ability to etch metal. Called galvanised metal-etching primer, this acidic bonding base adheres well to cast iron and promotes long-term finishes that won't flake or peel.
Not even galvanised metal-etching primer will adhere to unwashed cast iron surfaces. If any dust, grease or oils from your hands exist on the metal, poor adhesion results. Before you even think of adding a primer base to cast iron, scrub the surface with a water-based degreasing cleaner. This step is critical; don't skip over it, or flaking is likely to occur.
Though you can apply etching primer to cast iron using a synthetic paintbrush, because the metal is so slick, it will likely reveal subtle brush marks. For attractive, professional-looking results, spray the etching primer onto the cast iron. You can purchase spray cans of etching primer at most paint stores. Wear a respirator if you're painting in an enclosed, poorly ventilated area.