Metal Paint, Coating Metal Surfaces
Knowing which type of metal you’re working with is vital, since it will influence the performance characteristics of the coatings that you select for the job.
is a challenge for even the most seasoned professional. And while it is true that residential and light-commercial painters may not be as preoccupied with coating metal and rust prevention as their industrial counterparts, there are many places they encounter it in day-to-day work. Consider the handrails, staircases, pipes, metal door frames, ceiling joists, window frames, lamp posts, metal fences and garage doors — to name just a few metal surfaces – that can be found in typical residential or commercial settings.
There are two categories of metal—ferrous (containing iron) and non-ferrous (not containing iron). How do you know which you may be dealing with? Roy Colsten, Manager of Technical Services for Vernon Hills, IL-based Rust-oleum Corporation offers this simple method: “A ferrous metal is magnetic and a magnet would stick to it while on a non-ferrous metal a magnet wouldn’t stick,” he says.
Knowing which type of metal you’re working with is vital, since it will influence the performance characteristics of the coatings that you select for the job.
“It is important for a contractor to identify ferrous metals because they are corrosive and will rust,” says Andrew Kinnen, Architectural Product Manager for The Sherwin-Williams Company.
Corrosion takes place because of the minute differences in electrical potential between small areas on the metal surface which become anodic (a positive charge) and cathodic (a negative charge). When an electrolyte – that is, a conductor of electrical charges — connects anodes to cathodes, a corrosion cell is created.
Moisture on the metal surface is the most common electrolyte. In the electrolyte, a small electrical current begins to flow. The iron ions produced at the anode combine with the environment to form loose, flaky iron oxide – more commonly known as rust.
“Corrosion in the true sense of the word is the oxidation of a substance and steel is not the only metal susceptible to this process. Aluminum will also corrode, and when it does it produces a white powder called aluminum oxide. Non-ferrous metals are generally less reactive than ferrous metals,” says Carlsten.
Preventing corrosion on iron-containing metals can be accomplished by two methods: cathodic protection or barrier (blocking the electrolyte from the surface by using a paint system).
Cathodic protection, also referred to as sacrificial protection, is based on the knowledge that anodic metals have a greater tendency to lose electrons than more noble metals. “Metals are ranked in order of their susceptibility to corrosion, with the less noble, anodic metals being listed higher than cathodic metals. For example, zinc is more anodic than iron. When zinc and steel are connected in the presence of an electrolyte, the zinc becomes the anode in the corrosion cell and is slowly consumed, while the steel becomes the cathode and is protected,” says Carlsten.
This application is best suited to applications and environments where there is no acid present, because acid will react violently with the zinc and quickly disintegrate the coating.
Using a primer or paint high in zinc provides cathodic protection where small areas of steel and galvanized metal are exposed by scratches, drill holes or cut edges.
Coating the metal substrate with a primer and paint will provide barrier protection by isolating the metal from the environment and the potential electrolytes. “Several good paint systems are available for covering metal substrates. If ungalvanized, the surface should be cleaned and a good paint primer applied to the surface followed by latex paint topcoat,” says Kinnen.
“The one area where we see the most common mistakes made is when there is a small amount of rust on ferrous metal, and a contractor does not use a rust inhibitor primer,” adds Kinnen.
“Bare metal has to be covered with some type of rust deterrent,” agrees Carlsten. If rust is present on the substrate it must be removed before re-coating. Rust can be removed by scraping the surface with a brush, sanding, abrasive blasting or dissolving the rust away with acidic solutions.
“Traditionally the best way to handle rust is to remove it down to a clean metallic surface, however if this is not possible then scrape or wire-brush the metal down to a sound, rusted surface and apply a rust inhibiting primer which will soak into the rust,” says Carlsten.
All new ferrous metal surfaces should be primed with a rust inhibitor prior to applying a finish coat. Primers are designed to protect the metals and provide a base for the finish coat. The finish coat protects the metal from moisture migration as it also provides some aesthetic appeal. But no coating can completely protect against water vapor, so the function of a rust-inhibiting primer is to absorb and tie up the water vapor as it comes through the coating, thus preventing rust-forming reactions with the metal.
Rust inhibitor primers are available in both oil and water-based formulations. Sherwin- William’s All Surface Enamel Primer is an oil-based rust inhibitor that is relatively quick drying and can be followed by either latex or an oil-based trim paint. “We advocate using an oil-based primer only because of the sheer nature of water being applied to metal. But that is not to say water-based does not work in the correct applications,” says Kinnen.
“There are high-performance epoxy and urethane coatings that are designed for surfaces with small amounts of rust on them and can be used over marginally prepared surfaces. They will give good adhesion and resistance against corrosion by the nature of it being a product that applies at three times the thickness of enamel coatings,” says Carlsten.
In their efforts to reduce surface preparation time, some contractors to try their luck with rust converters, which turn hydrated iron oxide into other iron compounds. However, these coatings may still be long shots in the painter’s fight against rust.
Direct-to-metal (DTM) coatings are available that allow contractors to bypass the primer and get down to topcoating more quickly. “Since these coatings are half-primer and half-topcoat they are not as corrosion-inhibiting as the traditional two-coat system, but in some cases using two coats of DTM will work just as well as traditional systems,” says Kinnen.
“Coating galvanized metal requires a different approach and is probably the trickiest metal to paint,” says Kinnen.
Plating metal with a thin zinc coat makes galvanized metal. This is done to increase the service life of the metal by preventing corrosion, but it creates some unique painting problems.
Surface preparation is the key factor in achieving good paint adhesion on a galvanized surface. New galvanized metal is usually shipped from the factory with an oily surface to protect the zinc layers. This surface must be removed to allow adhesion of the primer to the substrate.
When galvanized metal is used on the exterior, this oily film will weather away in six months, making the surface paintable with a minimum of preparation (simply removing the dust or dirt from the surface readies the metal for priming). When galvanized metal is used inside or in sheltered areas protected from weathering, the surface oil should be removed manually before priming. This may be accomplished through the use of a solvent – such as naphtha or lacquer thinner — that is formulated with minimal oil content.
Imported galvanized metal usually has a passivator film that protects the metal from corrosion in the salt-water environment as the metal travels across seas or oceans on cargo ships. When exposed to the exterior, this passivator will weather away in six months; for interior use, it can be removed by etching the surface with a phosphoric acid solution.
After the galvanized metal is cleaned it is a good candidate for DTM coatings. As a general rule, alkyd or vinyl-acrylic coatings are not suitable, since they react with the zinc coating to produce a soapy film that causes the paint to slip off.
But most of the products for coating galvanized metal are acrylics. Sherwin Williams’ Opti-BondTM Multi-Surface Coating is a general-purpose, solvent-based coating approved for galvanized metal, forming the exception to the rule. “It’s one of the few solvent-based products available and is specifically designed with Portland cement in it for this purpose,” says Kinnen. Opti-Bond can also be used on aluminum siding or metal decking.
“In our mind, galvanized metal does not need a special primer coat,” says Kinnen. Latex acrylic paints are recommended as topcoats for galvanized metal. These fast drying, water-based paints have good adhesion, durability and weathering characteristics. This system is often used as a primer and topcoat and is suitable for both new and weathered galvanized metal.
Repainting galvanized metal can include spot repairing areas or total reconditioning of the entire structure. When spot treating, only areas free of old paints should be re-coated.
“Total reconditioning of a painted galvanized metal surface should be carefully evaluated,” says Steve Paulson of the Pittsburgh Paint Company. Obviously, the major problem with repainting galvanized structures is obtaining a good surface profile that will accept — and mechanically bond with — a new paint system. Roto peening may be necessary to produce an acceptable surface. Extreme care should be taken not to strip any more of the zinc coating than needed. The more zinc that is removed, the less cathodic protection is afforded to the base metal.
New aluminum should be allowed to weather at least six months before painting. The factory finish on aluminum may contain wax or silicone that could adversely affect paint adhesion.
“Our experience has been that aluminum comes through pretty clean, and usually only requires a direct application of latex. Don’t put an oil-based paint directly on galvanized or aluminum. That is bound to fail,” says Kinnen.
Aluminum exposed to the exterior has a tendency to chalk after weathering. Using soap, water and a soft scrub brush prior to painting should remove this chalking.
After the substrate has dried and all the chalk has been removed, latex or acrylic paint can be applied directly to the surface. If light chalk remains, prime with a chalk penetrating acrylic latex primer.
Being certain of your skills and knowledge will go a long way toward achieving success with coating metals. If you’re contemplating a project where painting metal will be a factor, you should investigate the many resources that are available.
“Prior to beginning a metal coating job, a paint contractor should have the technology and skill to complete the job. They should also ask manufacturers for recommendations of what to put on the surface to protect against the environment,” says Carlsten.
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