Zinc substrates

The main reason for the blistering, pinholes or craters is out gassing from within and under the zinc coating. There are several methods to minimize or eliminate these problems

  • Preheat the substrate to above the intended bake temperature to degas the zinc coating before powder coating.
  • Use slower gelling powders to extend out the ‘wet stage’ in the baking cycle of the powder, allowing more time for the gas bubbles to expel and the now molten powder to flow back out before the powder begins to set.
  • Add a degassing agent to the powder specified to slow down the gelling process (as above).

Poor adhesion of power coating to zinc substrates can be caused by the zinc galvanisers quenching the items in a strong chromate bath to cool the items down and to slow down the formation of a zinc oxide layer. This causes problems at the powdercoating stage as it is very difficult to remove this chromate coating to be able to etch and treat the zinc before powdercoating. Usually if it is explained to the galvanizer that the item is to be powdercoated, then they will usually air cool the items. But many informed zinc galvanisers now tend to use a very weak chromate quench bath which seems to be acceptable to the powdercoating industry.