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topic 37772

Zinc phosphate vs zinc plating as base for powder coat


Q. Our company supplies fittings to the fence industry and we now have the parts zinc plated then powder coated after. The powder coater doesn't like to paint over the zinc plating and wants us to use a zinc phosphate primer with a powder coat finish. Some of the product use includes getting banged around and I am wondering if one process is better than another. We would use hot-dipped galvanized if possible - but no powder coater wants to paint over it. Any help?

Cal Kooistra
- Grand Rapids, Michigan


A. I hope I could help you with your problem, this is the way we go about doing our powder coat ... first off they check for rust, if so they get dipped in a hot acid bath to dissolve any rust, then they get rinsed really well. Next they go to a 15% caustic soak cleaner which is run at 180 °F, then rinsed (really well) again. Next we go to a zinc phosphate immersion coating for approximately 2 minutes, then rinsed again, then they are rinsed in a hot water chromate rinse and air dried, then sent down to powder coat. This also puts a good rust inhibitor on the substrate.


Brian C. Gaylets
- Scranton, Pennsylvania, U.S.A.


A. I believe that the zinc phosphate will provide much better product for several reasons. The zinc phosphate covers the entire part, inside and out. The zinc plating will coat outside surfaces only (for example, the outside of tubing will plate, the inside is bare.) The zinc phosphate is an excellent paint base, the zinc plate is less able to promote adhesion because it is a smoother surface.

The zinc phosphate is a stand alone product. It is used to coat military weapons. Combined with the powder coating it should be much superior.

Iron phosphate is a standard coating used in the paint and powder coating industry as a pretreatment. Zinc phosphate is more corrosion resistant than iron phosphate.

Daryl P. Spindler, Sr.
Daryl P. Spindler, Sr., CEF
- Greenbrier, Tennessee

June 26, 2017

Q. Can we do Zinc Plating on part after phosphating?

Prashant Kumat
Vishwakarma Industries - DELHI, India

June 2017

A. Hi Prashant. Sorry, no. Zinc plating must metallurgically bond with the steel.


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Ted Mooney, P.E. RET
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