While metal paint primer is not always a requirement, it is always recommended, especially if the metal will come into any contact with moisture. Untreated metal requires primer to seal it from exposure before any topical paint may be applied. Some metals contain anti-corrosion materials, usually, an additional metal such as zinc is added solely to be dissolved in lieu of the primary metal.
Without the use of a primer, in many cases, oxidation will begin leading to rust and the eventual decay of the metal. In addition, the chemical makeup of paint is less binding than primer meaning it will flake off easier without a primer underneath it.
How Primer Works on Metal
Painting alone does not always provide a permanent coating. This is especially true for metal. Paint easily flakes off of metal if applied directly to the surface. To prevent this, the primer is used. It is like paint except it is designed to fill in the microscopic gaps in the material and stick to it. Applied over primer, paint adheres much better, for it easily bonds with the primer.
Primer also works as a protective agent. It resists moisture for materials that have no inherent water resistance. With most metals, without primer, not only would the paint flake, but oxidation would immediately begin once exposed to moisture.
Metals that Require Primer
Not all metal surfaces require a primer. If it does not, like with stainless steel, it means it already contains properties that allow it to prevent oxidation. Metal that is exposed to the elements requires a primer before it can be painted. In the home, metals that are commonly found include wrought iron, galvanized steel, and aluminum. These materials are used in furniture, railings, trim, gutters, barbecues, fences, and egress wells.
Many metal products available for resale purchase come pretreated, which includes primer and paint. Barbecues and gutters are two such examples. Wrought iron furniture, however, must be sealed if rusting is to be immediately halted. Aluminum products do not hold paint well without primer. It will also oxidize if not sealed properly. Tin is one metal that does not oxidize. It is used to coat other metals such as steel to prevent oxidation. In the case of tin as with pre-painted metals, primer is used primarily to provide a bonding surface for the paint.
Paint and Primer Types
Most importantly, a primer has to explicitly indicate that it is a rust inhibitor. If it is not, it contains nothing in it to prevent oxidation. Rust inhibitor primers such as those made by the Rust-Oleum company prevent oxidation by adding zinc to the primer which oxidizes in place of the metal. The zinc will have to dissolve entirely before the base metal begins oxidizing, a process that takes years.
Acrylic or latex metal paint is ideal for outdoor applications such as gutters, metal siding, and posts. Oil paint works better on furniture and railings, surfaces that receive more wear and tear. Primer is absolutely necessary underneath both types of paint even if the metal is already protected. As mentioned, this is to help the paint bond to the surface. Be sure you understand fully if there is a special topcoat requirement for the primer you use. While not all have this, some metal primers are made for use with particular paints.
Your next metal painting task will be much easier knowing whether or not priming is necessary. Choosing or working with a metal that doesn't require priming will save you time and money!