Powder Coating For Exterior Durability

Q. I have been approached to help layout some specifications, regarding a sizable order of 12-gage x 6-in heavy duty T-hinges for exterior durability. We are targeting a salt spray test (ASTM B117) of 500-750 hours and a QUV test (ASTM G53) of 1,000-1,500 hours.

The specifications are as follows: 

 

1. Molten zinc bath, substrate centrifuged to zinc thickness of 15-microns (Galvanizing)
2. Fine grain zinc phosphate conversion coating of 70 mg/sq ft with a dry clear organic seal
3. TGIC Black Polyester powder coat paint to thickness of 150 microns.

 

The paint thickness looks heavy, even for TGIC Polyester. Can that level of dry film thickness (DFT) safely be applied in just one coat? Any input that you can share would be greatly appreciated. B. K.

 

A. I see no problem attaining your corrosion standard and QUV resistance with your design. Between the galvanizing and the zinc phosphate you should attain up to 1,000 hours salt spray, fairly consistently. The QUV resistance should be a snap for most TGIC polyesters, as long as the pigments are UV stabilized. Red colors can be a problem though.

As for the dry film thickness (DFT) requirement of six mils, I question the need for such a thick coating, unless it will be used in a highly abrasive environment. If you insist on this film thickness, you can achieve this in one coat if you preheat the part to improve attraction of the powder to the part and overcome electrostatic rejection problems. However, I would recommend using a “low-flow” TGIC powder formulation to prevent runs, as a precaution.