Q: We work with aluminum and steel substrates, and the coatings are coming out different. The aluminum has a thinner mil build than the steel using the same manual gun. Any thoughts we could give to our customer?
A: This is an interesting question. Theoretically, there is no reason for powder to deposit differently on steel vs. aluminum. Both metals are conductive enough to allow an even deposition of powder. Hence, this begs a few questions.
As we both know, film thickness measurement on ferrous substrates uses magnetic force to quantify coating thickness, whereas film gauges for non-ferrous metals use eddy current technology. Some film gauges have both mechanisms in the same instrument, others do not. I wonder if perhaps the film thickness measurement is suspect. What do you know about the customer’s instrumentation and calibration technique/frequency?
If this is not the issue, there may be a difference in path to ground for the aluminum vs. the steel substrates. Are the same hooks used for both metals? Are they in different condition? Poor film build is most often caused by a poor ground. The electrons emitted by the tip of the gun have nowhere to go, and consequently the powder does not build.
So, what to do? Check out the instrument issue and verify the thickness measurements. Then investigate the continuity to ground. This is best accomplished with a megohmmeter (also known as a Megger). The resistance of the part to the ground must not exceed one megohm. If it does, then investigate all connections from the part to earth. Ensure that they are clean and making good contact.