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Trouble with powder coating and aluminum castings


We are having trouble getting our powder coating to stick to aluminum castings. Our process is:

1) Alkaline rinse
2) Fresh water rinse
3) Sprayed on wash primer (containing PVB, Resin, Phenol resin, Zinc chrome, Phosphoric acid) -- green
4) Powder coat -- black
5) Wet acrylic top coat -- black

The paint is chipping off all the way to bare metal. The green wash primer is sticking to the black chips that come off the casting. Any ideas how to fix this problem? Is the process good for aluminum? Is it possible that we need to bake the parts before step 3 to get the oils out of the casting?

Tina Goldstrom
- Clearwater, FL, USA


Dear Tina,

Nowadays it is considered bad practice to use any coating containing chromates due to H & S considerations.

However, wash primers have performed very well for 50 years when used over an aluminium substrate (formulated in early forties) providing that you have chosen the right one.

a) A true wash primer consists of PVB, Chromate pigment, Talc, Meths and water this is mixed just prior to use with a catalyst that contains phosphoric acid. Normally called two pack primers.

b) Then came along one pack type using phosphate pigments as part substitution for the chromate and these are called one packs mainly used in dip tanks.

Other modifications came along, epoxy, ureal, phenolics as part substitution for the PVB. Meths were replaced by butanol solvent in an effort to make the whole thing more fire proof.

You do not state which type of primer you use -- but as one of the early formulators of this type of product (US Navy was first) please try a good two pack wash primer made to the original spec and you can't go wrong -- provided the substrate is clean. Discard any primer left at the end of the day only apply sufficient primer to wet the surface of the metal-etch primers do not like a heavy coat applied as this can lead to poor adhesion.

Terry Hickling
Birmingham, United Kingdom

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