Most metal roof deck jobs are galvanized. Since you normally install a built up roof on top of the metal deck the color of the finish is irrelevant. Once in awhile though you have an uncommon condition in which you need a structural product that has an architectural finish. Your job requires a panel that has a galvanized substrate, can span a long distance, has structural engineering, and requires a nice looking paint finish. It would seem like such a product would exist, but it does not.
Unfortunately, metal decking is not made that way. When this happens you will have two choices:
- Order metal decking that is galvanized and also has a factory applied primer coat.
- Paint over the galvanized metal decking
Painting Metal Decking That Is Galvanized And Prime Painted
The best solution is to plan ahead and order a metal decking that is galvanized and has a factory applied primer coat. Normally, the top side of the steel will remain galvanized and the underside will have a primer coat. However, it is possible to get metal deck with a double primer coat. In this instance you will have a galvanized finish with a factory applied primer applied to the top and bottom of the panel. Just keep in mind this is not something that you will find in stock. Allow three to four weeks lead time.
Factory galvanized and prime painted metal deck is meant for larger jobs of at least ten thousand square feet. When you have a larger job the increase in price is relatively minimal. In this case the price will increase 5% to 15%, but the higher cost of the steel will be offset by the lower labor cost of painting the metal deck.
As the project gets smaller the cost of the metal deck will increase. If you have a really small job of under one thousand square feet it could literally cost ten times as much. When the job is closer to ten thousand square feet than it will be a minimal cost difference. Something in the middle, let’s say three thousand to five thousand square feet would increase the cost by 20% to 30%.
Hopefully, you have planned ahead and allowed yourself three to four weeks for the material to deliver to your jobsite. Now it is time to discuss how to paint galvanized metal decking with a factory applied primer coat.
- Surface Preparation With A Factory Applied Primer Coat
- Painting The Primer Coat
Follow the instructions from the paint supplier and contact the metal deck manufacturer to verify which type of paint is compatible.
Ensure surface is clean, dry and free from contaminants such as; oil, grease, dirt or mud. Cleaning with water and mild detergent is a common practice. The paint applicator should follow the paint supplier’s recommended application procedure, especially with regard to temperature, humidity, and surface moisture.
Painting Galvanized Metal Decking
You cannot just apply paint to metal decking. If galvanized metal is not prepared and painted properly, the paint will adhere initially, but will quickly fail. It will tend to peel over time. Instead, there is a process that must be followed so that the paint will stick to the galvanized.
- Clean galvanized steel with a chemical degreaser
Galvanized metal is hard to paint because it's coated with a layer of oil that is designed to prevent white rust. You must get that layer of oil off of the galvanized prior to applying a primer coat. - Rinse and dry
- Prepare the galvanized steel
- Etch The Galvanized
- Apply A Primer Coat
- Paint The Material
Clean new galvanized metal surfaces with a chemical degreaser to remove any oil or and contaminants. Then use a clean, lint-free cloth and scrub the galvanized.
Rinse off the galvanized metal decking with water and let it dry completely for several hours.
Do you have a brand new galvanized or do you have galvanized that has been exposed to the weather and has white rust? When you purchase your metal decking request new steel that has not been exposed to the weather for a long period of time. Explain to the metal decking distributor or manufacturer that the metal deck will be painted and it is important to have a new, clean galvanized finish. If you have new steel you may be able to avoid this step. However, if the galvanized has any impurities than they will need to be removed.
Two Ways To Prepare The Steel: Sandblast vs. Scour
1. Sandblast the metal deckingThis is the only way to guarantee the removal of the chemical treatment without being too aggressive and also removing the galvanizing zinc coating. Hire a company that specializes in this. This is the quickest and simplest way to prepare the galvanized.
2. Scour Older Galvanized Steel Lightly To Remove White RustIf the galvanized has been exposed to the weather, you may notice a powdery or chalky film on the surface of the material. Buff out by using a 120-grit or finer, high-grit sandpaper. This process will take a long time. Using gentle circular motions, sand the steel until the exterior has a uniform appearance. Afterwards, dampen a cloth with warm water and wipe the surface to remove the dust.
After the metal deck has dried, dampen a cloth with white vinegar and wipe the surface down. Leave the vinegar on the material for 1-2 hours. This will give it plenty of time to eat away at the galvanized surface. The longer it sits, the better the etching. The acid in the white vinegar will act to etch the metal and get the paint to adhere better. Be careful not to miss a spot as you will get paint that peels from the galvanized in those areas.
Now that you have cleaned, dried, and etched the galvanized metal deck it is time to apply a primer coat. Purchase an exterior latex primer that is made specifically for galvanized metal. Follow paint manufacturers instructions and apply the primer coat. Let the primer dry according to the manufacturer’s directions. Test a small area to verify that the primer is adhering correctly.
Purchase paint that is the same brand as the primer coat. Read the directions on the container of paint and apply the paint per the manufacturer’s instructions. In most circumstances you will apply two coats of paint. Test a small area to verify that the paint is adhering correctly.
In a perfect world you will have both a large enough project and the required time to use galvanized and factory primed metal deck. This is a specialized product and it is recommended that you order the material from a metal decking distributor that specializes in this type of material. If your project is either really small or you need it quickly than you will not have a choice. In this case, you will have to field paint the metal decking. By following the steps above you can eliminate paint adhesion issues.
If you need to learn more about metal decking then check out our learning center.
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