Mass Painting Steel Bolts

Question: We manufacture Grade-8 bolts and apply a black oil finish.


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We manufacture Grade-8 bolts and apply a black oil finish. Our customers are asking for a dry black finish that has similar corrosion protection to mechanical zinc plating but without the cost. Phosphate and oil comes close to working, but does not have enough corrosion resistance and must be cleaned before painting. Is there a low-cost, low-thickness, off-the-shelf water-based black paint available that we can put over zinc phosphate? If so, where can we get it? L. M.


All kinds of fasteners and other small parts have been coated with various color paints including black. I have no data showing relative performance characteristics of various corrosion protecting treatments such as mechanical zinc plating and phosphate and oil. However, if the corrosion resistance phosphate and oil provides to steel bolts is close to that of mechanical zinc plating, painted zinc phosphate should be better. Yes, there are low-cost, low-thickness, water-borne black paints that can be applied over zinc phosphate. They can be applied by spraying, dipping or dip-spin methods. You can even apply them by electrocoating. I hope that by wanting low cost you don’t mean you will buy junk paint. Suppliers of water-borne paints are listed in the 2005 PF Directory and Technology Guide (www.pfonline.com/suppliers.html).