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August 18 - August 22, 2014

How do you measure wet film thickness on shop-applied organic zinc-rich primers to accurately predict dry film thickness? Our shop applies a great deal of OZ primer, and we have found that WFT readings with a standard gauge are not accurate predictors.

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Selected Answers

From Larry Muzia of Exceletech LLC on August 22, 2014:
 The wet gage measurement is not exactly accurate  because the metallic zinc particles do not allow the two ends of the gage to make direct contact with the substrate. They are a solid, and if zinc is under the edge, it will prove to be inaccurate. The is the case with both organic or inorganic zinc filled coatings. I agree that an agitated pot or very low speed agitator in the bucket if spraying airless is a must. Remember to keep the speed low to avoid inducing a vortex sucking moist air into the product if the humidity is high. As for accurately measuring the wet film, the use of a wet film gage will give you an approximate reading, although it will not be 100 % accurate. Experience is never out of style, and, typically, these materials dry within hours, allowing a confirmation using a dft gage.

From Steve Brunner of WPC Technologies on August 19, 2014:
I find it odd that a wet film gauge does not yield accurate predictions. Given that, I recommend consistency and a practice board. On the former, thinning the same to a consistent viscosity will keep the solid content nearly identical from batch to batch. Then use a practice (pattern) board for the applicator to determine gun pattern, adjust the application equipment and helps out the applicator with the arm speed. I have seen a lot of applicators who use practice/pattern boards and always end up being spot on with the DFT. The last issue is use of an agitated pot. It has been my experience that zinc-rich coatings will settle in rapidly, thus the need to keep agitated. If not agitated, the coating will have an inconsistent solids content.  On this last thought, this could be the problem and the easiest fix.

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Tagged categories: Coating / Film thickness; Dry Film Thickness (DFT); Wet film thickness

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