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Metal preparation prior to applying powder coating paint

Q. I am in process of developing paint specifications for material handling equipment, and need information as to how the metal is to be cleaned and treated prior to applying the power coat paint. Do you recommend a primer and a finish coat?

Thanks,

William Schiesz
consultants - Hoover, Alabama
2003

A. With powder coating, you don't usually need a primer coat. I would suggest though, that a thorough pretreatment regimen be applied before the powder. This would include cleaning/degreasing, phosphatizing, and sealing. The phosphate coating will provide better opportunity for the powder to adhere to the substrate. Hope this points you in the right direction.

Dan Brewer
chemical process supplier - Gurnee, Illinois
2003

Q. What do you mean by sealing? What do you use to seal the metal after cleaning/degreasing?

Liz Key
manufacturer - Port Jervis, New York
August 4, 2009

A. Hi, Liz. In this context, Dan means a chromate, or more likely a non-chromate replacement "final dip" that is recommended by the manufacturer of the phosphatizing solution as suitable for finishing the pretreatment sequence. In some cases it may even be just DI water.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 21, 2009

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Q. After hot degreasing, rinsing in water twice, acid pickling in 40% strength HCl, rinsing in water twice, hot phosphating, rinsing in DM water and finally post passivation, and kept in dry-off oven for 15 min at 150 °C temp...

Should the powder coating be done within any specific period? Can we keep it for 12 to 15 hours in a dry place and then powder coat. Will it affect salt spray time which should be 300 hrs?

Dorai Raj
- Chennai, Tamil Nadu, India
March 9, 2012

March 15, 2012

A. Isn't phosphate a hygroscopic crystal?
If you keep pretreated material for any long time after drying, will the crystal not absorb moisture?
And then during curing at say 200 °C, won't that water come out of the crystal, explosively, causing surface irregularities in the curing coating?
We used to use zinc phosphate, and had a rule that if the metal could not be coated within one hour or coming out the dryer, then it went back into pretreatment again.
We don't use phosphate now, but still have that rule.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

Q. We are having a debate between our powder coater and our casting house as to what is the best cleaner to use prior to painting. Our castings are chromated. The painter wants to use MEK verses alcohol. We are seeing some paint chipping issues with the samples not cleaned with MEK. Is this a possible reason or is alcohol good enough?

Eric

Eric Carlson
- Woodridge, Illinois
March 13, 2012

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