Inter-Coat Adhesion Problems

Q. What causes problems with inter-coat adhesion between epoxy primer and urethane topcoat?— L.H.

A. This is a common problem with all types of dual-component coatings. There are two types of adhesion, and understanding each type should help you better identify the source of the problem. 

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First, adhesion is the ability of dissimilar particles or subsequent surfaces to bond to each other. In this case, it can be broken down into two common types: chemical adhesion and mechanical adhesion.

In chemical adhesion, subsequent coatings literally melt into the previous coating. For example, after you spray a nitro-cellulous lacquer on a substrate and it has dried, you can go back in an hour or a week and spray an additional coat of the same lacquer without having to do any additional surface preparation work (dirt or contaminate aside). The coating literally will melt into itself. This, however, is not what is happening in your current finishing system.

For mechanical adhesion, the primer needs to be sanded to promote the subsequent coat’s adhesion to the fine-scratched or etched surface. In essence, you are breaking the surface tension with thousands of scratches to give the next coating a surface it can hold on to. It should be noted that the aggregate size for the sanding is very important. A 400-grit sandpaper may not produce enough of a scratch pattern to promote proper adhesion. Typically, coatings on metals are applied with greater mil thicknesses, so a more aggressive 180- or 220-grit sandpaper would be recommended.

Two part-finishing systems typically incorporate a catalyst, which promotes a crosslinking process that ultimately makes the coating extremely hard at the surface level. In essence, the product will not adhere to itself because of this surface tension and must be sanded with the proper sandpaper first before a subsequent coat(s) can be applied.

I probably would not adopt a wet-on-wet approach for your finishing process for the simple reason that you may affect the chemical properties of each coat. The purpose of a primer is to give you a good, solid anchor for the next coat to adhere to. If it is wet and you then apply another wet coat, you may affect the ability of the primer to perform the way it was designed.

I think the mechanical adhesion approach is the best remedy for your problems. Apply the primer coat, allow it to cure, and then properly sand it before applying the topcoat.