Telegraphing Phosphate Coating

Question:

I am the paint line supervisor. We paint steel parts in our shop. To comply with local air quality regulations we started to use high solids paints. Shortly after we started using them, we began having problems. We are using a high-solids paint over an iron phosphate pretreatment. The phosphate is applied in a three-stage washer. After painting, we are experiencing some telegraphing of streaks which appear to be low gloss acidic streaks. We have tried an alkaline cleaner and had what appeared to be high gloss alkali streaks. Do you have any suggestions on how to solve this problem? R. B.

Answer:

Acid streaking of iron phosphate pretreatments should be hidden by paint. Generally one can only see the streaking when the coating is too thin. When the film thickness is increased, the streaking will disappear. In the case of some high-solids paints and certain water-borne paints, telegraphing of streaks at higher film thicknesses can be due to the rheology of the paint. This is caused by pigment settling as the paint is drying and it results in an apparent thin coating in the streaking areas. If increasing the film thickness does not help, you can try to eliminate streaking by hanging the parts on the conveyor differently.