Three dirty words

Rust. Oxidation. Rot. Here's how to clean up on them.

How protective coatings are applied can greatly affect their ability to keep moisture and corrosion at bay, say corrosion prevention experts from leading coating companies.

“Although coatings can change the appearance of the building materials for an aesthetic affect, their primary job is to protect the material from corrosion,” says David Johnson, product manager at Martin Senour.

He says corrosion prevention starts in the design phase of a building project. Architects and engineers must specify the right materials and the design should help the structure shed water and allow adequate air movement to keep the surface as dry.

Corrosion happens when moisture gathers on a surface and chemically attacks the material, says Bob McGovern of Crown North American Professional Products.

“When surfaces get dirty, they can collect moisture,  encouraging corrosion. In ferrous metals it appears as rust, in concrete, it appears as spalling and in wood, you’ll find rot,” he says.

Prevention is protection
The best corrosion protection is prevention, says Johnson. Once a surface begins to corrode, all oxidized material must be removed to halt the corrosion process.

Fastidious surface preparation is a must. Most coatings protect by creating bond with the surface. “The surface must be clean and roughed up so the coating can create an effective physical bond with the surface,” says McGovern.

Poor surface preparation is the biggest mistake
“Poor surface preparation is the biggest mistake contractors make when trying to control corrosion,” Johnson says. “If you don’t completely remove all of the residue or contamination, the coating cannot properly adhere to the surface and protect it.”

Poor surface preparation can also be a problem on concrete. “In tilt-up construction, it can be hard to get proper coating adherence because the release agent wasn’t adequately removed before applying the coating. The release agent creates a ‘non-stick’ type of surface so the coating can’t adhere properly.

“In up to 90 percent of the complaints we get about coating failures, the cause can be traced back to inadequate surface preparation. That’s why surface preparation is so important.”

Application tips
The experts say contractors often err by applying too much or too little coating to the surface. “When it comes to application, more is not better,” says McGovern. “If you apply a heavy coat and it dries, it may crack over time. This creates a channel that allows moisture to infiltrate the surface. It harbors the moisture and the corrosion process continues, often unseen,” he says.

Johnson says contractors and building owners often use coatings to hide the surface. “They often want to ‘spruce up’ a facility, and a coating can do that. But you must achieve coverage that produces a .001" to .002" thickness for best results. We hear of applicators thinning coatings in hopes of getting them to go farther. This over-reduction doesn’t do anything to help prevent corrosion because once the solvent or carrier evaporates, it leaves behind a layer that is too thin to protect the surface. It can also create sags and runs or may not cure properly if the coating is reduced too much.”

Look to new technology
Volatile organic compounds (VOCs) are becoming a greater issue in areas with ozone or smog concerns. Coatings that contain solvents can contribute to air pollution, so some areas restrict their use. Here, water-based coatings are an option, says Johnson.

“Water-based coatings have improved substantially. They can create a tough, attractive layer of protection,” he says.

They are also less toxic and less flammable than solvent-based coatings.

Contractors who apply coatings regularly may benefit from using the new high-volume, low-pressure (HPLV) technology, says Johnson.

“These new tools can apply coatings so that up to 95 percent of it reaches its target. Conventional air and airless sprayers get at most 40 percent of the coating on the target.

“The technology is slightly slower than conventional systems, but there are greater savings in materials.

“Although field-applied coatings are not as closely regulated for VOC emissions as those applied in production processes, a contractor who prefabricates may be affected by VOC regulations. The HPLV technology may help meet those regulations,” he says.

Coatings storage and use
Often, aerosol cans of coatings get relegated to the spot behind the truck seat or in a temporary tool shed where temperatures can reach over 120 F or fall below 0 F. It’s not good for the product and when it comes time to use it, you may be disappointed.

“Check the label for storage instructions. In most instances, they should be kept in a dry area where the temperature does not reach extremes,” says McGovern.

“Temperature can affect the propellant and even the solvent of some products. For some products in the summer, storing it in a service truck’s toolbox may be too hot.”

When using an aerosol, McGovern says that in most instances, shaking it until you hear the marble inside rattle is all that is needed to adequately mix the product.

“Even on viscous products like corrosion preventers, shaking the can for three or four seconds after you hear the marble inside of the can is enough to agitate the product for correct application,” he says. But additional shaking doesn’t hurt.

Published in the July/August 2001 issue of Contractor Tools and Supplies magazine.

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