Field Painting of Galvalume� Sheet

Prepainted Galvalume and bare (unpainted) Galvalume sheet are both recognized as premium products in the metal building industry. Although neither product requires field painting in normal use, there are reasons why field painting may be desirable:

• The coatings, either the topcoat and primer or the Al-Zn coating, have been damaged during shipping, storage, on-site fabrication or erection.
• The building has been erected in an overly aggressive environment, such as severe salt water marine, and premature corrosion is occurring.
• The coating has been severely damaged by an overdriven fastener, drill bit or other mechanical means.
• It is desired to change the color of prepainted Galvalume sheet.
• The customer has decided to paint bare Galvalume sheet for aesthetic purposes.

As with other metallic-coated or prepainted sheet steels, Galvalume sheet can be easily field painted by following established procedures and using readily available paints. A clean, dry coating, whether bare or prepainted, is crucial to provide a good base for paint. Careful selection of the paint system is necessary to insure its compatibility with the metal or painted coating and to promote adhesion. Note, however, that the application of any field-applied coating may have warranty implications. Please review with your building panel supplier.

Surface Preparation:

The surface to be painted must be clean and dry. Dirt should be removed by washing with water. Grease and oil may be removed from Galvalume sheet with a mild detergent (e.g., one-third cup Tide� per gallon of water) followed by a thorough rinse. Degreasing solvents such as mineral spirits may also be used. For more stubborn surface stains, stronger cleaners such as Oakite 84M produced by Oakite Chemical (50 Valley Road, Berkeley Heights, NJ, 07922, (908) 464-6900) can be used on bare Galvalume sheet. All of the above must be followed by a thorough rinse of the Galvalume surface since most cleaners are mildly corrosive. Abrasive materials, such as wire brushes and steel wool, should not normally be used on coated steel products. Their use may remove the protective coating and decrease the life of the product. One exception is preparing rusted areas: careful wire brushing is then necessary to remove the rust. The damaged coating should be removed back to areas of tight adhesion and the edges of the existing coating feathered. When there is severe rusting, grit blasting or power sanding may be necessary to completely remove the rust. Remember: use only the least damaging method to attain a clean surface.

Primers:

No priming is required to touch-up or repaint prepainted Galvalume sheet. For best results, the supplier of the original paint system should be consulted to assure compatibility with the field-applied system. When painting bare Galvalume sheet, priming is strongly recommended.

Etch Primers: When using etch primers, only two-pack etch primers, also called butyral wash primers, are recommended. They are commonly used under a variety of finish coats and never by themselves. When the components are mixed, the pot life is limited to about 6 to 8 hours and a thin translucent film should be applied, preferably by spray. Brush application is suitable for small touch-up areas. Etch primers can be overcoated after 30 minutes air drying and should be covered preferably within 3 to 4 hours. Since they are water-sensitive, they should not be applied in damp or excessively humid conditions.

Zinc Dust/Zinc Oxide Primer: Oil-based zinc dust/zinc oxide primers are two-component systems most effectively used as a primer or self-priming finish coat for roofing, guttering, etc. They are available from many paint companies as Federal Specification TT-P 641, Type 1 or in proprietary formulations. When used without a separate top coat, they will have a gray matte appearance and tend to chalk excessively outdoors. Resistance to chalking can be improved by the addition of aluminum flake pigment to the zinc dust paint (approximately 2 lb/gal).

The first stage of preparation is to thoroughly mix the oil-based medium so that the pigment is evenly distributed throughout the mixture. Then pour off a little of the oil-based medium and mix it with zinc dust to make a paste. This paste form is the best method for adding the zinc dust to the oil-based medium. The manufacturer's recommended proportions of zinc dust to oil-based medium must be followed. Once the zinc dust/oil medium paste is added to the remaining oil medium and thoroughly mixed, the complete mixture should not be stored in sealed containers, since pressure can build from the spontaneous generation of hydrogen.

Under good conditions, zinc dust/zinc oxide primers should dry for a minimum of 48 hours outdoors or 72 hours indoors before applying a finish coat.

Water-Based Acrylic Primers: Some water-based acrylic primers containing an anti-corrosive pigment give excellent performance, provided the finish coats are also water-based formulations. Overpainting with oil-based paint considerably reduces the adhesion of the entire system.

Finish Coats:

Without Primer: As discussed in the primer section, zinc dust/zinc oxide with aluminum flake pigment may be used effectively as a self-priming finish coat.

Relatively high gloss water-based, self-priming 100% acrylic paints are available for direct application to Galvalume sheet. The color white should not be used in severe environments because it contains considerably less anti-corrosive pigment than other colors. As with the water-based acrylic primers, these paints should not subsequently be topcoated with oil-based paints.

Where good quality water-based, 100% acrylic paints are used on Galvalume sheet, etch priming is not necessary to obtain adhesion, but should be used to upgrade corrosion performance.

With Primer: For normal environments, Bethlehem has found good corrosion performance with a zinc chromate butyral wash primer in combination with a solvent-based siliconized acrylic topcoat. Water-based systems may also be available that would perform well. The paint manufacturer should always be consulted to determine specific product recommendations for good performance with Galvalume sheet.

For severe marine and industrial exposure, the more protective double finish coats such as polyurethanes or vinyls should be used. In applications where Galvalume sheet will come into contact with soil and groundwater, two coats of a two-pack coal tar epoxy paint are extremely effective. Edge priming is not essential.

Asphalt-Based Fiber Coating: Another system that has been tested and used successfully for repair painting of Galvalume sheet is Uniflex 500�, an asphalt-based, fiber aluminum system available from Kool Seal, Inc., 1499 Enterprise Parkway, Twinsburg, Ohio, 44087, (800) 321-0572. This coating is normally applied directly over the Galvalume sheet and is silver gray in color. Severely damaged areas that have developed significant rust may require spot priming -- consult Kool Seal for specific recommendations.

General Rules: Regardless of which paint system is selected for repainting or repairing Galvalume sheet in the field, the following general rules should be practiced.

Read the manufacturer's instructions and observe them explicitly. Thorough mixing is essential. It is not advisable to use different brands of primers and finishing coats in conjunction with one another. Do not overpaint water-based paints with oil or organic solvent type paints. At all times, it is important to avoid excessive paint film thickness such as may occur in the valleys of formed roofing panels. Where extensive areas are to be covered, spray painting can lower cost while giving acceptable
results. The paints used, however, must be formulated for this purpose. Good safety practices are imperative. Many of the paint systems available can give off toxic fumes and cause reactions when in contact with the skin. Proper safety equipment should be used with adequate ventilation at all times.

Guidelines for Field Painting Bare and Prepainted Galvalume Sheet

• Wipe or wash away all loose dirt and debris
• Use detergent or solvent to remove oil and grease
• Rinse thoroughly

• Remove any surface stains with approved cleaners
• Completely remove all rust to bare metal using the least damaging method practicable
• Wipe or wash away all loose dirt and debris
• Use detergent or solvent to remove oil and grease
• Rinse thoroughly

• All surfaces should be completely dry

• Not necessary
  

• Siliconized acrylic
• Uniflex 500
• Zinc dust/zinc oxide
• Acrylic
• Polyurethane or vinyl (severe marine/industrial)
• Coal tar epoxy (contact with soil or ground water)