Q: I have a question concerning the relationship between pretreatment chemicals (currently using a zirconium-based pretreatment in a six-stage system with an alkaline cleaner in stage 1) and powder coat thickness. Can the condition of the bath play a role in how thick the dry film thickness is on a part? I don’t think so, but others in my organization feel like there is some cause/effect on bath and thickness.
A: You pose an interesting question, and your instincts are correct. To avoid confusion, there is more than one way to consider this question.
First, does pretreatment affect the measurement of powder coating thickness? The answer is yes. The pretreatment is a physical layer on the surface of the substrate and therefore must be considered when measuring film thickness.
This issue is easily managed by standardizing your film thickness gauge to read zero on the pretreated substrate. Otherwise, you’ll be measuring the coating thickness and the pretreatment layer. In reality, the pretreatment is a rather thin film (typically a couple tenths of a mil), so it doesn’t significantly skew the thickness measurement. Precision is precision, however.
Second, the question you are asking deals with the affect pretreatment has on film build. Does pretreatment influence film build during application? Does the quality of the pretreatment cause the film build to be higher or lower? My experience says, no, it does not.
To affect film build, the pretreatment would have to affect the continuity of the part to ground or insulate the substrate surface to lower the conductivity. I don’t think the layer of pretreatment is thick enough or insulative enough to cause either of these phenomena.
I hope this helps settle your dispute. If you really want to nail down this conclusion, you could run an experiment by coating an unpretreated part vs. a pretreated one using the same application conditions and see if you observe a difference.