Scum on Waterborne Paint

Q. I am the manufacturing engineer at my plant. I am also responsible for the finishing line. We make children’s domestic swing sets, see-saws and other outdoor playground toys. Many of the parts are fabricated from thin walled steel tubing. We form the parts, then paint them by dipping in a water based enamel. The parts are cleaned in an iron phosphate washer. Sometimes these parts seem to raise scum to the surface of the paint tank. Is it possible that the material that is surfacing is carbon from the steel tubing? What can I do to solve the problem? V.M.

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A.Since you didn’t give me a complete description of your cleaning and painting facility, I must make some assumptions. First, I assume you weld some of the steel parts. Some welding residue is what is called smut. The scum floating on the surface of your waterborne paint tank could result from such things as: 1) Carbon or smut floating to the surface from the immersed steel parts. 2) Cleaning chemical residue that is not completely rinsed off the steel. 3) Residue from oily soils that are not completely removed by the cleaner stage.

In any event, I suspect your pretreatment operation does not have the ability to remove all the soils. To verify this, check the condition of your cleaning and rinse stages. If they are within specifications, change to a stronger cleaner. If, on the other hand ,you determine the scum results from welding smut, add a desmutting stage to your cleaning operation.

Since your playground equipment is used outdoors, I’m surprised you don’t pretreat using a zinc phosphate. The zinc phosphate would provide far more corrosion protection to the steel surfaces than the iron phosphate. This change would result in longer product life and fewer customer complaints.