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topic 46160

Can we do powder coat on top of powder for rework



A discussion started in 2004 & continuing through 2017

(2004)

Q. Hello People,

I will really appreciate your help, just want to place this question: What is the best way to rework the powder coating (electrostatics)? or a website where can I consult.

Kindly,

Eliseo [last name deleted for privacy by Editor]
- Juarez, Chihuahua, Mexico


April 28, 2009

Powder Coating Complete

A. You've found the right website, Eliseo, with 50,000 Q&A's about metal finishing. But you need to post specifics of your situation -- because there are so many types of parts, so many substrate materials, so many types of thermoplastic and thermoset powders, and so many individual situations. By "rework" do you mean apply touchup paint or do you mean apply a second layer, or completely remove defective powder coatings and start over? Thanks.

Maybe start with letter 1152, "Touching up powdercoat" which introduces a number of different approaches. Good luck!

Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


April 29, 2009

A. Eliseo,

What is it you are trying to do? If you would like to strip the coating completely off, you may want to try a fluidized sand bed or a molten salt bath, you can blast it with abrasive media, you can dip it in stripping solution or depending on the type of powder you can use paint thinner.

Then again, you might just want to quickly sand it and paint a second coat and rebake it.

Rodrigo Salinas
- Juarez, Mexico



(2007)

Q. I have cured powder parts that for what ever reason need to be reworked. Can I just put on another layer of powder without stripping off the first layer? Are there down sides in doing this?

Dave. Stewart
QA - Bowling Green, Kentucky


(2007)

A. In my experience you can do this once only. You get two chances to get it right, no third.
But... this depends on the competence of the first coat. If its well adhered, to a well prepared and pretreated substrate, and provided that you are using the same type of powder, and that you sand the surface of the first coat to get intercoat adhesion. Then you should be able to recoat.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland



(2007)

A. Hi,

Applying a second or even a third coat of powder is common practice when alloy wheel manufacturers, OEM, powdercoat -i.e. primer, aluminium then finishing off with a transparent polyester powder. You will just have to evaluate for yourself to see if a second coat is possible. This is because some powders are more self-limiting than others as to the amount of powder you can apply. You could heat the object prior to applying the powder. This method will always work as the powder does not depend upon the electrical charge applied to the powder but tends to melt as it settles upon the hot metal substrate.
Terry

Terry Hickling
Birmingham, United Kingdom


(2007)

A. Hi.
As per my experience you pass your components in 100 ° C temp and when parts get a temperature more than 70 °C immediately coat the second layer and then pass it to oven as per your specification. I'm sure that you'll get a good adhesion of both the layers and rework would be successful.

Kuldeep Singh
Wet process employee - Pantnagar, Uttrakhand, India


(2007)

A. It depends on the COATING REQUIREMENTS.
But here is some advice according to my experience;
Analyze residual coating
Strip part
Sandblast
Keep it dust free
Process coating
Verify requirements

Jose Castellanos
- Minneapolis, Minnesota, USA


(2007)

A. It's always better to get it right the first time but we've all had rejects. Whether or not you can recoat the parts depends a lot upon the actual powder your using and the operator applying it. Some powders just don't recoat well and some operators don't have the experience and/or ability to setup the equipment properly to make acceptable recoats. In any event, you need to sand between the coat or you will not get acceptable longevity of the topcoat being applied to the base coat. Peeling will occur if the base coat has been totally cured before you have put another topcoat over it. With proper sanding and powder application, you can achieve great success, however you may reduce the powder manufacturers performance statistics for the powder quality by the addition of more powder than is called for by the manufacturer.

bob utech
Bob Utech
Benson, Minnesota

A Guide to High Performance Powder Coating
by Bob Utech
[link is to Amazon]


(2007)

A. Yes, You can do recoating, First Scancheck the first (prime) coat for any corrosion or loose paint, Roughen it with sandpaper, Pre heat the component.

During coating reduce the charge of the gun and increase the powder flow, not all guns support this the GUN should also be supportive, use DIPOLE guns as they are the best for this act.

Regards,

O.SHANKAR KHANDELWAL
RENU - COIMBATORE, Tamilnadu, INDIA


June 26, 2009

A. Yes you can recoat the compound. In two different ways you can recoat:

1) Preheating the compound to certain temperature (100 °C) and directly you can apply the powder. Here no gun setting is required or anything required. You can coat as such your specified gun setting. But practically it can be carried out only in big compound because preheating small compound each time and coating is quite difficult.

2) For small compounds, just put sandpaper, reduce the voltage of the gun and increase the powder flow.

ArunBau
- Coimbatore, TamilNadu, India



March 25, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. How is the reworking method for thermosetting powder coated paint?

Nihal Pathirana
- Ja Ela, Sri Lanka


Fundamentals of Powder Coating

March 2014

Hi Nihal. We appended your inquiry to a thread on the same subject, and maybe it will help you. But what do you mean by "reworking"? Do you mean a small touch-up, or attempting to put a new color onto a previously powder coated part, or do you mean stripping and starting over because the first coat is blistered, orange-peeled, or otherwise defective? Please detail your own situation rather than casting questions in the abstract. Thanks!

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey




February 3, 2016

Q. Hi, I have just had my Mongoose bmx bike frame powder coated and very happy with the finish but not happy with color, do I have to sand blast again or can I change colors without without sandblasting? I would appreciate any advice on this. Thanks Aussie Ray

Raymond Peters
- Melbourne, Victoria, Australia


June 6, 2016

Q. If a part was already powder coated. Can I add a second layer of powder coating but with more texture. what are the pros and cons

JOSH SNYDER
- IRVINE, California, U.S.A



August 30, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Getting parts powder coated and we have noticed the thickness varies from 35 - 50 micron. I contacted the powder coater and he said he would coat them. We require 60 -80 micron. We are using a Matte finish powder. Is re-coating product satisfactory from a performance and appearance point? What are the options?

FRANK KAISER
BUYER - SYDNEY


August 2016

A. Hi Frank. You are the customer and you specified the 60-80 micron thickness for some reason, so you must know the performance and appearance requirements. It is up to you to determine if the reworked parts are satisfactory. But we appended your inquiry to a thread where a number of experts have said that yes indeed it can often be done satisfactorily. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



October 8, 2016

Q. Sirs,
I had a question about powder coating in full-matting; the clients use this coating. Reworking cause second coating peel off, first coating is normal. I checked my client's devices which oven temperature is normal. What reason cause second coating peel off? I look forward to answer by you, thanks.

Rishana Chen
- Taiwan,Tainan


New powder coating over old

June 28, 2017

Q. Hello Everyone My first post here!

I have a large powder coating contract with a local city government. There are about 100 pieces largest is 32 8 ft long 400+ pound park benches and a bunch of 22 and 32 gallon trash cans. These item were powder coated new 15 years ago. 90% of the powder is still attached very solid with a mill thickness between 2 and 5. My plan is to sand blast any and all rust and loose powder coat off and powder coat over the old powder. This is the client's idea to keep the cost as low as possible. I have never powder coated over old powder before ...is there any special pretreatments required? Should I have the old powder coat tested for compatibility of the powder going over the top?

46160-1a   46160-1b  

Randal Moreland
- ROMNEY, West Virginia


June 30, 2017

A. My Friend,
You are on the edge of a very deep precipice.
This product is 15 yrs old.
You do not know the formula of Powder previously applied.
There is no upside for a contract powdercoater taking on a refurbishment job without taking complete control of the process. Imagined Cost saving by your client could very well turn into real losses for your company as undoubtedly you would be held responsible for performance.
There are simply too many risks involved with your intended process.
Fully blast or chemically strip & blast then apply a compatible restoration agent to the most heavily corrosion affected areas (Epirez comes to mind), sand off, pretreat as appropriate, then apply a quality thermoplastic powder.
That way you will avoid law suits and have a product your customer and yourself can be proud of.
Hope this helps,
Bill

William Doherty
Trainer - Newcastle Australia


June 2017

thumbs up sign Thanks William. Those were exactly my thoughts but I did not have sufficient personal experience to confidently state that the proposed approach was a terrible idea.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


July 2, 2017

thumbs up sign Thank you for your time and input.
I thought this is what I would hear back. I've been telling myself the whole time this is not a good thing. I have always stripped everything to bare metal.The best thing is I haven't signed the contract yet.

Randal R Moreland [returning]
Mountain State Powder Coating - ROMNEY, West Virginia USA


July 9, 2017

A. I agree with what's been said.
We would not undertake such a contract. The coating you apply might be good, but it's adhesion is relying on the old coating (assuming you get good adhesion to that old coating). And the old coating is failing already (actually I think its done pretty well to last 15 years with no other corrosion protection system underneath).
Start from bare metal and take full control.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland




October 19, 2017

Q. Hi to all!

I have a question ... I have now a coating line with different colors, sometimes have been events where I have coated by mistake with X color a piece, but it need to be recoated with other color.

My question would be, can I just recoat other color over the first layer (different color)?
or what would be the right rework process?

Substrate (CRS), all parts are flat; I have a restricted pretreatment process.

Roberto Rey
- Juarez, Mexico


October 21, 2017

A. If you are confident of the original coating (and that should be if you did it), then adhesion between the coast is your issue. You could do something as simple as sanding the original coating and over coat it.
If the 1st coat is fully cured, there will be no bonding due to the coast curing together, so sanding is probably best and easiest. We sometimes do that, mostly successfully.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland



November 4, 2017

A. We have done this many times without issue. Most times, it's as easy as making sure the surface is clean. We use windex and tack cloth. As long as the gun settings are correct for recoating, it typically covers very well. In some cases though, the colors are important. For example, we needed to coat some black signs silver. They turned pink in some areas. Another example, we coated some doors yellow on one side, white on the other. Any yellow over spray under the white had bled through.

Kayla Fagen
production manager - Cedar Rapids, Iowa, Usa



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