Intercoat Adhesion

We fabricate steel enclosures, on a contract basis for equipment manufacturers. To solve corrosion problems, we upgraded our paint finish by switching to a more corrosion-resistant epoxy primer. 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?


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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? K. L.

 

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. 

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