Wet Adhesion Test Problem

Q. We have problems with the wet adhesion test of epoxy primer 515K011 over colorless chemical conversion coating. We test wet adhesion according to FED STD 141 method 6301. The test pieces for checking adhesion are 2024 T3 aluminum alloy, clad or unclad. We have some questions about the wet adhesion test: 

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1. Is it necessary to cure paint coatings seven or 14 days before the wet adhesion test?
2. Is it necessary to protect the edges of test panels before the water immersion test? S. G.

A. After reviewing Federal Test Method Standard 141C, Method 6301.2, I see no requirement for curing the coating before testing. There is also no mention of taping or otherwise protecting the panel edges before testing. On the other hand, it is good laboratory practice to do both.

When I ran the paint lab at Westinghouse Research Laboratories, we aged panels before any testing and protected panel edges before the wet tests—high-humidity, salt-spray and immersion. Furthermore, these procedures were written into Westinghouse paint specifications. All panels painted with both air-drying and baking finishes were aged for five days at 50°C. This was done to ensure that the finishes were fully cured.

It is important to protect panel edges before performing the wet tests. Owing to the possibility of low edge film thickness, especially on sprayed panels, corrosion could occur at the edges, propagating inward and thereby ruining the test. At first, we protected panel edges by taping those undergoing the wet tests using vinyl electrical tape. We found this procedure to be tedious. Later, we dipped panel edges ¼ inch into a fast air-drying zinc chromate pigmented epoxy primer. This procedure was five times faster. We never had an edge failure on protected panels.