Adhesion to Latex Paints

Question:

We are a manufacturer of corrugated steel deck products. We manufacture (by roll forming) steel roof and floor decking. I hope you are familiar with this product, if not, go to one of the “Big Box” stores and look at the ceiling and the corrugated roof ( on top of the joists). You will find the type of products we make. We roll form the product from steel coils that are either uncoated galvanized or painted with a gray primer paint at 0.3 mil DFT.

Our coating process is like most coil coating lines except on a much smaller scale. We clean the steel, rinse, apply the paint, oven bake to a PMT of 190°F and then recoil the steel for future use. Our primer paint is an acrylic latex—water base and is formulated for post forming applications.

Here’s my concern. Our painted product (once installed ) may or may not receive a top coat of paint. For many years the top coat of choice has been alkyds, oil base or some form of solvent coating. Lately, we are seeing latex paints being specified and are getting more complaints because of incompatibility. Some latices crawl, fish eye or do not achieve acceptable adhesion. I have to assume some of them work (we don’t get calls then) but are hearing this complaint more often. We used to get the field painter to change his coating but we are seeing more reluctance to do that.

Our coating supplier says that latex coatings are still in their infancy and have not come up to the level of solvent coatings relative to wetting out, bite and adhesion after cure. Are you familiar with this topic or have knowledge or information that I can get a hold of for guidance on this issue? There are a million types of latex coatings out there and we cannot be compatible with all of them but the field guys seem to think this is our responsibility and not theirs. We feel the painter should be responsible. Hence the problem. G. W.

Answer:

I see nothing wrong with your process. There may be a problem with your material. There may also be a problem with your coating supplier. Latex coatings have been widely used commercially for 50 years. He may think it will take another 50 years for them to move from infancy to maturity.

Paints applied to metal coil and strip stock that is to be roll formed is formulated with a slip agent that comes to the surface during cure. This slip agent allows the coated metal to be roll formed without galling and peeling. This same slip agent can act as an interference coating on the painted surface. This is, it interferes with adhesion. Some solvent-borne paints are compatible with paints containing slip agents. That is, they will wet the painted surface. Most waterborne paints do not wet as well as solvent containing paints. Hence the problem. On the other hand, some latex paints wet better than others. For example, alkyd-modified acrylic latex paints wet better than unmodified acrylic latex paints.

Your coating supplier should know these things and should be able to give you a coil coating paint with a lower level of slip agent or a different slip agent that will be more compatible with latex paints. This will take some effort on his part. If he is unable or unwilling to make these changes, you should look for another coating supplier