Film Thickness vs. Paint Use

Question:

I run an anodic acrylic enamel electrocoat paint system. I have a set standard of seven tenths of a mil of dry film build. I have specs of square foot surface area coverage per gallon of paint. What would change the square foot coverage if I did nothing different to the process of pretreatment, electrocoat, post rinse and cure time. I am having a negative paint use variance, and I am maintaining my standard of 0.7 tenths of a mil of paint. My paint use has increased more than 15%. How can this happen? W.W.

Answer:

Could this be another case of the “Innate Perversity of Inanimate Objects?” Perhaps not. Among things that can affect film thickness when almost everything else is equal are pretreatment coating thickness, crystal size of the phosphate coating and percent solids by volume of the coating material. It is also possible that since the phosphate coatings are electrical insulators, the pretreatment coating thickness and crystal size may have an effect on the organic film thickness in some electrocoat systems. Depending on the measurement method, the phosphate film thickness and crystal size difference may go unnoticed while the apparent film thickness is the same. The percent solids by volume can also affect film thickness. If the solids are low, the paint use will increase to compensate.