Phosphate Bleed Through
Q. I am the paint line supervisor at my plant where we make a line of sheet-metal cabinets. The metal is iron- phosphate pretreated and then coated with a one-coat enamel. The high-solids paint supplier discontinued producing our paint, and we now are using another one of its products. All of a sudden we began having problems with the appearance of our coated products. The irregularities in the phosphate coating are telegraphing or bleeding through the paint when the high-solids coating is applied.
Our pretreatment consists of a two-stage wash followed by a clear-water rinse. Our water source is a well on our property. The water is at 17 grains as used in all three stages. The cleaner/phosphate stage is at 1.8 oz/gal and runs at 140ºF. The clear-water rinse is continuously overflowing. How can I resolve this problem? K.R.
A. It sounds like your pretreatment system is in good working order. Since the only thing that was changed in your finishing system is the paint, it is most likely the cause of the problem of bleed-through or telegraphing of the phosphate coating and not the pretreatment. The problem could be that your high-solids paint may have a hiding power that’s too low, it may be applied at too low a film thickness or a combination of both. Improperly formulated waterborne as well as conventional paints having low hiding power can cause the same problem. I suggest you investigate using other high-solids paints from your present supplier or from another supplier.
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