The importance of Surface Preparation in Coatings Projects
Corrosion – the one word that strikes fear into the hearts of property owners, maintenance supervisors, and design engineers across the globe. Nothing is immune from corrosion. From the apex of human engineering to the most basic of substrates, we are engaged in the never-ending battle against Mother Nature’s will.
At the core of every coating project, the principal goal comes down to two components: the elimination of corrosion, or the prevention of it. Whether you are relying upon galvanic protection most commonly found with zinc coatings, or a barrier coating that encapsulates the substrate, the longevity of your coating is directly correlated to the surface preparation prior to application.
An old colleague once said, “if you don’t have time to do it right, you better have time to do it twice”. Nothing could be truer when it comes to coatings. Improper surface preparation is the leading cause for coatings failure. When you factor in the material costs, labor, shutdown times, equipment rental, and other ancillary expenses, the need to get it right the first time is extraordinary.
Coatings adhere to the substrate one of two ways: through a chemical or mechanical bond. If relying upon a mechanical bond, it is imperative to achieve the necessary surface profile and level of cleanliness listed in the specification. Not achieving the required surface profile, either having too large or too small of a profile can cause problems.
Creating too low of a profile may result in the coating may not have enough “tooth” to form the mechanical bond with the surface. This can result in a loss of adhesion down the road. Having a surface profile that is too aggressive runs the risk of pinhole rusting. If a coating can only be applied at 2 mils dry film thickness (DFT), and your surface profile is 2.5 mils, the peaks of that surface profile may protrude beyond the film of the coating leaving bare steel to atmosphere.
Not all projects require the substrate is prepared down to bare metal. Specifications may permit “tightly adherent” existing coatings to remain. Tightly adherent is generally considered to be a coating that cannot be lifted with a dull putty knife. It is important to understand these terms and follow the written specifications.
Chemical bonding has its own set of concerns that also need to be addressed. The main cause of coating failure in a chemical bonding coating is the presence of surface contaminants. Soluble salts, oils, and other contaminants that are not removed from the surface prior to application will come back to haunt you. These contaminants generally need to be removed through washing or the use of detergents. Blasting or abrading the surface does not typically remove surface contaminants – it only redistributes the contaminants on the substrate. Blistering, “fish eyes” and delamination of the coating are the most common symptoms of a substrate that had residual surface contaminants present during the application process.
Just as a house is only as good as the foundation it sits on, the same is true for coatings. Proper surface preparation allows you to maximize your coating service life while simultaneously lowering your coating life cycle costs.