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"How can I powder coat without Zn phosphating?"


I am new to this forum. I am trying to powder coat MS sheets just after Shot Blasting + Preheating. It is good in adhesion test but It doesn't seem to withstand salt spray test. Is there any way that It can withstand salt spray without Zn-Corro or Zn-Phosphating. I am currently using Pure Polyester based powder paint as product is intended to be used outside.

Hammad Akram
Student - Pakistan


Dear Hammad,

Yes there are several other materials that can be used in preference to Zn Phos (what is Zn Corro?).

1) Epoxy primer or Zinc Rich Epoxy Primer.
2) Waterborne non-toxic primers.
3) Etch primer (wash primer) but the industry is trying to eliminate this type of product due to the Zinc Chromate content (environmental issues).

Why are you shotblasting, to remove rust etc? This results in a larger area that is susceptible to rusting.
Why pre-heat? Standard practice would be: clean substrate, apply powder and bake.

Polyester is very good for coating items to be sited outside, however it is no better than any other powder once the applied film is damaged. This leads to rust creep that can only be prevented by the application of a barrier - Zn phos or one of the above listed alternatives.
ASTM B117 [affil. link to spec at Techstreet] : approx.
No pre-treatment 24-48 hours
Zn Rich 2000 hours
Waterbourne primer 1000 hours
Zn Phos 750- 900 hours
Iron Phos 24-48 hours
Etch Primer 350-500 hours.

Terry Hickling
Birmingham, United Kingdom


Thank you very much Terry for your reply....!

I am using Preheating to get the paint thickness about 80-- 100 microns. My problem is that I don't have phosphating setup and I want my powder coated samples to qualify salt spray test... :) Once if I know that there is no other method then I will develop Phosphating apparatus setup.
"Zn corro" is the commercial name of under coat offered by "Jotun" Powder coatings.. But it is quite expensive...

Hammad Akram
- Pakistan


Dear Gentlemen,

For corrosion protection as well as better adhesion of powder can be achieved by using single stage Iron/zinc phosphate by brush or by cloth, if you do not have any proper tank facilities. The normal salt spray test of 50-60 hours can be stand by this process.

Navin Maheshwari
- New Delhi, India


Dear Hammad,

Sorry for the delay, but August is our (UK) annual holiday time.
Try lowering the Kv on your powder gun to about 40-50 KV -- you should be able to achieve 80-100 microns cold spray, but check that your earthing as a poor one will have an adverse effect on the film build of your coating.

Terry Hickling
Birmingham, United Kingdom

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