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Proper surface preparation, plus the right choice of primer and paint, is key to a long-lasting paint job.

When it comes to painting the exterior of a residential, light commercial or commercial building, nearly every structure, regardless of whether it’s constructed of wood, brick, stucco, vinyl or other material, is probably going to have some metal to paint, ranging from gutters and downspouts to railings and light posts.

To help ensure that these metal surfaces will “wear like iron,” The Rohm and Haas Paint Quality Institute offers the following recommendations for painting two of the most common metal surfaces: ferrous and galvanized. For tips on painting aluminum, see the Spring 2001 issue of Professional Painter.

Ferrous Metals

Ferrous metals are those that either contain or derive from iron. They are commonly used in the manufacture of castings, fabricated sheet steel, and wrought iron. With the exception of stainless steel, all of these metals will rust, which can eat away at the metal as well as spoil its appearance and undermine any applied coatings.

Rusting can start almost immediately when unprotected ferrous metal is exposed to rain, snow, dew or moisture in any form. Your objective, then, is to stop any rusting that has begun and to keep moisture and air from interfacing with the metal after painting.

Accordingly, ferrous metals call for very thorough and meticulous surface preparation. Anything less may seriously compromise the appearance and durability of the finished paint job, not to mention the integrity of the metal itself.

Preparing the Surface

The first consideration when preparing a ferrous metal surface in a noncorrosive atmosphere for painting is to remove any loose rust that is present, as well as any peeling paint. On smaller jobs, use a chisel-style scraper and a hand-held wire brush. Use the scraper to take off heavy rust and loose paint, then follow up by wirebrushing the surface to remove as much of the rust residue as possible. It is not necessary to remove every bit of rust and take the surface down to bare metal, but rather to remove as much rust as these methods will allow.

On larger jobs, power wire-brushing or disk sanding with aluminum oxide paper is effective. Whether you use hand tools or power equipment to remove rust, be sure to wear personal protective equipment, including eye protection and a good dust mask.

After wire-brushing, the surface will be covered with small particles of loose rust and dust, which should be removed before any coating is applied. Brush these particles off with a softbristle brush, then scrub the surface with a detergent-and-water solution, followed by a thorough rinsing with clean water.

Surface preparation should not be omitted just because an iron or steel surface is new. New ferrous metal often has mill oil on it or small amounts of rust that are not readily visible. Not removing these before applying a coating could result in premature failure of the paint job.

Timing of Priming Is Critical

Once a ferrous metal surface is free from rust and other impurities, priming should priming trimbe done as quickly as possible. This timing is vital because rust can begin to re-form on iron or steel if the surface is exposed for as little as a day or two. If that happens, you’ll have to prepare the surface again.

When painting ferrous metal, it is important to apply a top quality metal primer because it must perform two vital functions: provide the bond between the topcoat and the metal, and inhibit corrosion. This is a point you should not compromise.

porch railings When priming ferrous metal:

DO: Use a quality exterior rust-inhibitive primer.

DO: Apply the primer at the recommended spread rate in order to achieve adequate film thickness, which directly impacts corrosion resistance.

DO NOT: Thin the primer before application, unless recommended by the manufacturer.

DO: Consider applying a second coat of primer for maximum corrosion resistance.

Top quality acrylic latex corrosioninhibitive primers work well in applications where the metal is not exposed to heavily corrosive atmospheres, such as acidic or salt air. Unlike oil-based or solvent-based primers, they can be applied immediately after cleaning the surface, even if it is still slightly wet. Zinc-rich and zinc chromate epoxy and alkyd primers are more appropriate for more highly corrosive settings.

Selecting the right topcoat is also important when painting ferrous metals. A high quality acrylic latex paint is generally a good choice because it can last as much as two to four times longer than conventional alkyd paints without serious cracking or fading, has a much quicker drying time, and is easier to handle and clean up.

Galvanized Metal

Galvanized metal is iron or steel that has a thin coating of zinc on it to help prevent rusting, and it is commonly used for gutters, downspouts and flashing.

If the galvanized surface is new or unweathered, wash and thoroughly rinse it before painting. This step is necessary to clean off any zinc chromate or residual oil left from the galvanizing process, which otherwise can interfere with adhesion of the paint.

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