Q. I have cold-rolled steel (CRS) metal tubes that I need to coat with a clear powder finish. The customer likes the industrial look of the raw steel under the clear powder. My problem is after powder coating, some of the parts show rust underneath the powder. What can be done to stop the rust from showing, or is there a way to take the rust off before powder coating? The material is not rusted during part manufacturing, before going to the powder coater.–A.R.
A. The problem you are seeing is resulting from residual mineral salts on the tubing. This is tough because anything you do to treat the tube before coating will change that industrial, raw steel appearance. You could use mechanical abrasion or chemical treatments to get rid of reactive mineral salts, but it will change the look of the tube. You could try cleaning with an alkaline cleaner and then a DI or RO rinse, which will give you a very clean surface with minimal impact on the look. It does not add any corrosion protection to the steel however, so you need to dry the parts thoroughly and get them coated right away. The parts will eventually rust if they are used outside and exposed to rain and sun. Try some parts and see if they show rust when exposed to extreme humidity. Be sure your customer understands that you have no real corrosion protection, however.