Solving Intercoat Adhesion Problems

Q. I am a manufacturing engineer in a plant that produces small appliances. We are having trouble with a baking enamel. When a part is rejected because of a paint defect, we try to recoat it, but the second coat won’t adhere to the first, causing us problems. What is the best type of polymer for a baking enamel that allows it to be recoated with excellent adhesion?

A. Small appliances that are finished using enamels have excellent mar resistance, because slip agents are added to enamel formulations. However, when they are recoated, the slip agent can interfere with adhesion. The first thing to do is investigate this possibility as a root of your problem. Baking enamels that use most resin polymer types can be recoated with excellent intercoat adhesion if they don’t contain a slip agent. Enamels that offer the best adhesion, both substrate and intercoat, are formulated using epoxy resin polymers. However, when applied over enamels formulated using slip agents, even they will have poor intercoat adhesion. Another possible cause for intercoat adhesion failure is that the original enamel is under-cured.  

Q. We fabricate steel enclosures, on a contract basis for equipment manufacturers. We were having corrosion problems. To solve the problem, we upgraded our paint finish by switching to a more corrosion-resistant epoxy primer. We topcoat this primer with the same enamel we have been using for several years. Tests run by one of our customers indicate that while the salt spray resistance has doubled, the humidity resistance has decreased markedly, showing blisters within 100 hours. We never had a blistering problem before. What can we do?

A. Your problem is a classic case of poor intercoat adhesion. Many coatings, notably epoxies, have a maximum limit on their “time to recoat” because they cure to an extremely chemical-resistant state. I suspect your new primer is such a coating.

To solve the problem, either apply the topcoat within the specified “time to recoat’ limit or try one of the following:

1. Lightly sand the primer to increase surface roughness.
2. Apply a tie coat which is an intermediate coat of paint that will bite into the primer’s surface. Some paint suppliers recommend the use of a tie coat in these cases.
3. Modify the topcoat by adding a strong solvent that will bite into the primer.

One of the aforementioned solutions will not only improve intercoat adhesion, it may even increase salt spray resistance.