Weld Drag Marks

Q. I am a sales rep for a painting equipment manufacturer and picked up a copy of Products Finishing while sitting in a customer’s waiting room. Regarding the question by B.F. (January 2011) concerning spot weld drag marks: While your answer was certainly correct, the question may still not be completely answered.

The term “bleeding” reminded me of a past experience with a custom coater who had problems with discoloration of the finished, white, baked, epoxy enamel film. Since the product was a flat sheet, 24 sq inches with four welded studs, manufacturing required sliding the pan across the lower (paten) electrode from one stud to the next, depositing copper on the part. These deposits reacted with the enamel to cause unwanted stains. Also, the deposited copper was not removed from the metal during normal cleaning and phosphatizing, and is usually not visible until after baking of the film.

Two suggestions to alleviate the problem: First, remove the source by using a harder alloy electrode or stud support surface to prevent contact with the electrode while not welding. Simple spring-loaded ball bearing supports might work. Second, remove the offending copper deposit chemically. Alternatively, a lower baking temperature for a longer time can be successful, and recoating will certainly cover up the problem, but both of these courses are impractical. P.P.

A. Thank you for furthering my education by writing to Painting Clinic. I welcome and appreciate your comments and solicit the same from others.