painting metal

8/4/2019 Painting Metal

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Proper surfacepreparation, plus the

right choice of primer and paint,

is key to along-lasting paint job.

W hen it comes to painting theexterior of a residential, lightcommercial or commercial building,nearly every structure, regardless of whether its constructed of wood, brick,stucco, vinyl or other material, is prob-ably going to have some metal to paint,ranging from gutters and downspoutsto railings and light posts.

To help ensure that these metal sur-faces will wear like iron, The Rohmand Haas Paint Quality Institute offers thefollowing recommendations for paintingtwo of the most common metal surfaces:ferrous and galvanized. For tips on paint-ing aluminum, see the Spring 2001 issueof Professional Painter .

Ferrous Metals

Ferrous metals are those that eithercontain or derive from iron. They arecommonly used in the manufacture of castings, fabricated sheet steel, andwrought iron. With the exception of stainless steel, all of these metals willrust, which can eat away at the metal aswell as spoil its appearance and under-mine any applied coatings.

Rusting can start almost immedi-ately when unprotected ferrous metal isexposed to rain, snow, dew or moisturein any form. Your objective, then, is tostop any rusting that has begun and tokeep moisture and air from interfacing

with the metal after painting. Accordingly, ferrous metals call

for very thorough and meticulous sur-face preparation. Anything less mayseriously compromise the appearanceand durability of the finished paint

job, not to mention the integrity of themetal itself.

Preparing

the SurfaceThe first con-

sideration whenpreparing a ferrousmetal surface ina noncorrosiveatmosphere forpainting is to

remove any loose rust that is present, aswell as any peeling paint.

On smaller jobs, use a chisel-stylescraper and a hand-held wire brush.Use the scraper to take off heavy rustand loose paint, then follow up by wire-brushing the surface to remove as muchof the rust residue as possible. It is notnecessary to remove every bit of rustand take the surface down to baremetal, but rather to remove as muchrust as these methods will allow.

Painting


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7Professional Painter Spring 2002

On larger jobs, power wire-brush-ing or disk sanding with aluminumoxide paper is effective. Whether youuse hand tools or power equipment toremove rust, be sure to wear personalprotective 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 beremoved before any coating is applied.Brush these particles off with a soft-bristle brush, then scrub the surface witha detergent-and-water solution, followedby a thorough rinsing with clean water.

Surface preparation should not be

omitted just because an iron or steelsurface is new. New ferrous metal oftenhas mill oil on it or small amounts of rustthat are not readily visible. Not removingthese before applying a coating couldresult in premature failure of the paint job.

Timing of Priming Is Critical

Once a ferrous metal surface is freefrom rust and other impurities, primingshould be done as quickly as possible.This timing is vital because rust canbegin to re-form on iron or steel if thesurface is exposed for as little as a dayor two. If that happens, youll have toprepare the surface again.

When painting ferrousmetal, it is important to applya top quality metal primerbecause it must perform twovital functions: provide thebond between the topcoat and

the metal, and inhibit corro-sion. This is a point youshould not compromise.

When priming ferrousmetal:DO: Use a quality exteriorrust-inhibitive primer.DO: Apply the primer at therecommended spread rate inorder to achieve adequate film

thickness, which directlyimpacts corrosion resistance.DO NOT: Thin the primerbefore application, unless recommend-ed by the manufacturer.DO: Consider applying a second coat of primer for maximum corrosion resistance.

Top quality acrylic latex corrosion-inhibitive primers work well in applica-tions where the metal is not exposed toheavily corrosive atmospheres, such asacidic or salt air. Unlike oil-based orsolvent-based primers, they can beapplied immediately after cleaning thesurface, even if it is still slightly wet.Zinc-rich and zinc chromate epoxy andalkyd primers are more appropriate formore highly corrosive settings.

Selecting the right topcoat is alsoimportant when painting ferrous met-als. A high quality acrylic latex paint is

generally a good choice because it can

last as much as two to four times longerthan conventional alkyd paints withoutserious cracking or fading, has a muchquicker drying time, and is easier tohandle and clean up.

Galvanized Metal

Galvanized metal is iron or steelthat has a thin coating of zinc on itto help prevent rusting, and it is com-monly used for gutters, downspoutsand flashing.

If the galvanized surface is new orunweathered, wash and thoroughlyrinse it before painting. This step isnecessary to clean off any zinc chromateor residual oil left from the galvanizingprocess, which otherwise can interferewith adhesion of the paint.

Metal so that it wears like iron


3/4Professional Painter Spring 2002

Apply a quality exterior acryliclatex corrosion-inhibitive primer forbest performance. However, if thereare no signs of rust, a top qualityexterior 100% acrylic latex paint can beapplied to new galvanized metal with-out a primer.

Oil, alkyd and vinyl latex paints,however, should never be applieddirectly to bare galvanized metalwithout first applying a corrosion-inhib-itive primer. Without an appropriateprimer, these coatings can lose theiradhesion to this substrate, sometimes inless than a year.

If the galvanized surface is weath-ered and unpainted, clean and rinse off any dirt and remove any rust with a non-metallic scouring pad. Then apply a

corrosion-inhibitive metal primer and topquality exterior 100% acrylic latex paint.

If it is previously painted, carefullyremove any rust that is present and allloose and peeling paint with a wirebrush. Avoid cutting through the layerof zinc galvanizing. Then wash thesurface with a detergent-and-watersolution, rinse it thoroughly, and applya corrosion-inhibitive metal primerfollowed by a 100% exterior acryliclatex paint.

How Many Coats?

When it comes to the number of coats of primer and paint to apply tometal, the cardinal rule is to follow therecommendations of the coating manu-facturer. In general, however, one coat of primer is normally sufficient, althoughapplication of a second coat insurescomplete coverage and maximumprotection in demanding situations.

Two coats of a topcoat are generallyrecommended rather than a single coatfor two reasons: increased overall thick-ness of the coating for betterdurability, and elimination of the possi-bility of any pinholes extendingthrough the coating.

In addition, always apply primerand paint to metal in thick coats for

optimum durability and rust resistance.Thats because the thicker the coat, theless chance of moisture penetrating thepaint and reaching the substrate.Brushing is fine for small applications,while rolling and airless spraying arebetter for covering large areas.

Direct-To-Metal Coatings

Another option when paintingmetal is Direct-To-Metal (DTM)coatings. These coatings are designedfor direct application to metals withoutthe use of a primer. They function asboth primer and topcoat in one product.

DTM coatings can be used for avariety of applications, are available insemigloss or gloss formulations, andare now offered in high performancelatex products. To ensure properprotection of the metal, two thick coatsare usually recommended.

Regardless of whether you use aDTM coating or a primer-topcoatsystem, it is not difficult to get years of reliable service from a metal paint job.The keys are taking the time to properlyprepare the surface, promptly primingthe surface, and applying thick coats of the right type of coating.


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Why Its Important toPrepare the Surface

Good paint performance depends on good adhesion,and paint adheres best to surfaces that are clean andsound. Thats why contractors make sure the surfaceis in this condition before they start.

Taking shortcuts on surface prep can cause even thehighest quality paints to fail prematurely. In fact,according to experts at The Rohm and Haas PaintQuality Institute, inadequate surface preparation isthe

single greatest cause of premature exterior paint failures.

Thus, just as a builder needs a solid foundation toconstruct a house that lasts, so too must a paintingcontractor start with a sound surface. It is the first stepto a durable, long-lasting exterior paint job.

Benefits of a

Properly Prepared Surface Better adhesion the paint is less likely to

blister, flake or peel.

Better uniformity the paint has a moreconsistent color and sheen or gloss.

Better hiding no show-through from thesurface that is being painted.

Increased mildew resistance the paint

wont be marred by unsightly black or brownfungal growth.

Increased corrosion resistance, in thecase of painting metal results in a longer-lasting paint job because of better protectionagainst rusting.

painting metal

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