Blisters Around Weldments

Question:

We are coating cold-rolled welded steel parts with waterborne paints. We are using an iron phosphate. The problem we are having is blistering around the weld areas. We need some suggestions. H. J.

Answer:

A few things that could cause your problem at weldments are poor pretreatment, rough weldments and porous weldments. If your pretreatment is inadequate, by-products of the welding process are not being removed. If the weldments are too rough, the cleaner cannot impinge on the entire surface and therefore will not remove all the oily soils. When the parts are baked the paint blisters in those areas where the paint has bridged across the soils. If the weldments are too porous, the phosphatizing solutions are trapped in the interstices and cause blistering of the paint film during the baking cycle. It is also possible that because of the aforementioned porosity, rinse water could be trapped in the weldments and not be removed by the dry-off oven. These suggestions should give you something to look for in the search for the cause of the blistering problem.