If you try to add a painted finish directly over the top of any sort of metallic surface, noticeable flaking will begin after a few weeks. Metal is a notoriously poor candidate for adhesion. The proper primer will improve metal's adhesive qualities. Unfortunately, ordinary primers have little effect on metal, because the material is nonporous and very slick. Learn which primers are best suited for the particular metallic surface you're working with, or finish failure will prove unavoidable.
The Wrong Primers
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Water-based acrylic and latex primers are not appropriate for metallic surfaces. If you try to use one of these bases on any sort of metal surface, finish failure will eventually result. Though the finish may appear durable after initial application, heavy chipping and/or peeling will begin within weeks or months of application.
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Etching Primer
Metal-etching primer is well-suited for smooth metal surfaces which are prone to finish failure. Professionals use sanding techniques to roughen smooth surfaces, such as tile, vinyl, acrylic and fiberglass. Unfortunately, slick metal is too durable for such tactics. Metal-etching primer has an acidic base that allows it to etch slick metallic surfaces. It is especially suited for galvanized metal which is already highly-resistant to rust.
Iron Oxide Primer
Iron oxide primer is suited for metallic surfaces subject to rust. While galvanized metal is coated with a layer of zinc that inhibits rust, untreated metals such as wrought iron are prone to oxidation. Iron oxide primer inhibits rust and is appropriate for any untreated metal surface subject to moisture.
Zinc Chromate Primer
Zinc chromate primer is similar to iron oxide primer in that it bonds to metal and inhibits rust. Unfortunately, zinc chromate is highly toxic and is typically not appropriate for use by inexperienced painters.
Considerations
No matter what type of primer you choose, it will not bond to an unwashed metal surface. Before applying any sort of primer to metal, thoroughly wash the surface with a water-based degreasing cleanser, or finish failure will loom likely.