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Brown-black powder coating problem: white or silvery specks

Q. We have brown color powder we use in powder coating. With fresh powder there are no white spots but when we start to use reclaimed powder, white spots start to come at surface. Suggest what is the issue.

Powder manufacturer accepted that it is due to metallic pigmentation. They have to change their metallic pigment.
Is there any change in process?

Powder coating application manager - Goa India
July 20, 2023

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Q. WE HAVE PROBLEMS WITH BLACK POWDER COATING. AFTER CURING YOU CAN NOTICE SILVER SPECKS AND SOME KIND OF DEBRIS. We have cleaned the booth, cleaned the oven, serviced the compressor, changed the spray gun, and checked the degreasing system -- but we can not cure this problem. It only occurs with black.

Usha Patel
powder coating - United Kingdom

A. Sounds like all you have left to try is sifting the paint before you use it. Use an 80 mesh screen. Don't use a rotary sifter, use a vibrating sifter, if the paint is coming in dirty you will know it after sifting about half a box.

Sheldon Taylor
Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina

simultaneous replies

? Is your powder clean?

John Martin
- Wales

A. Usha, is it possible the lot of black paint you are using is contaminated? Have you tried spray black powder from another container or lot? That may help you to isolate the source of contamination.

Roy Nuss
Trevose, Pennsylvania, USA

A. Dear Sirs,

You do not state the type of powder that is causing you the problems but from the information you have - silver specs - one guesses that it is an automotive quality, semi matte black polyester. If this is the case the silver looking specs are in fact the additive that is post added to improve the flow/fluidity of the powder. The bits; Sheldon has already covered - sieve before use. Whose powder is/was it? A more detailed explanation can be supplied if needed.

Terry Hickling
Birmingham, United Kingdom

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Q. Welcome to the club. I have exactly the same problem with gloss black and have tried the same remedies. I bought new powder and more powder from another manufacturer. Same problem so I don't think it is the powder. I am inspecting my finish with a 30x hand microscope. I find these specks to be anywhere from less than one mil up to about 2 mils. Some of them sparkle and almost look like tiny craters down to the substrate, or flecks of a reflective substance such as some kind of crystalline flake. Can't really tell at this magnification.

I then inspected a manufacturer supplied sample panel. I was amazed to see the same thing but to a slightly lesser extent. The difference is that I will have a few more visible to the naked eye (and unacceptable on the work) whereas the manufacturer's panel specks are less noticeable to the naked eye.

I went on and examined panels of other colors. I again found some of the same crater-like specks but they are not noticeable to the naked eye.

This problem is driving me nuts! It is apparently widespread so I would be very grateful if someone could shed some light on it.

Larry Johnson
Larry Johnson
- Key West, Florida

simultaneous replies

A. Usha, something else you might want to consider(unless you're already doing it) is to use color specific equipment. A good practice is to have different hoses(maybe even hoppers and pumps) for blacks, whites, light colors, dark colors and metallics. Especially if you run metallics(just guessing here, you didn't mention other paints that you use) because there isn't any way to get all of the flakes out of a hose that has had a metallic in it, unfortunately it won't solve the trash issue.
I ran a system once that had different hoppers, pumps and hoses for each color, that way all that had to be cleaned was the booth and guns. Never had a contamination problem.

Sheldon Taylor
Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina

A. Dear Larry,

The apparent silver specs is the reflectivity of the additive added to improve the flow/fluidity of the powder. The name of this additive is a grade known as precipitated Aerosil. In very bad cases the problem is caused by insufficient mixing this into the batch of powder after post addition. Alternatively, it is possible for production get the quantities wrong and move the decimal place to the right by mistake - thereby putting much more in by a factor of ten. Without this material your powder would not flow and may eventually cling together forming large lumps.

Terry Hickling
Birmingham, United Kingdom

Q. It appears that sieving is the solution and an 80 mesh screen is recommended.

I am concerned that it may not remove what is affecting my coating. The most visible specks to the naked eye are about 1 mil and up. The mesh chart I have ( a 7 mil particle size for 80 mesh and I'm wondering how that will help.

Larry Johnson
Larry Johnson
- Key West, Florida

A. Mr. Johnson, The sifting suggestion was basically for Mr. Patel's debris problem, I don't think that I would try to sift out the silver specs, like Terry pointed out they could be part of the paint. If they are I wouldn't imagine them being visible after curing using typical visual inspection criteria (24" to 36" distance under fluorescent light). If the specs are causing rejectable product though, the paint manufacturer must be involved in the solution.
It might be a good idea to tell your paint manufacturers rep or techs your inspection method and see if they can offer any suggestions.

Sheldon Taylor
Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina

A. We are also facing this problem. Probable causes as per my observations are ...
1.Particle size of additive i.e. Aerosil
2.Particle size of fillers
3.Particle size of matting agent (in low gloss products)
4.Impurity in raw material
5.Improper dispersion.
6.Gelation during extrusion. We are using 125 mesh

Santosh Dhuri
sandipak - Tiska, Goa, India

A. Without any direct details your problem comes from powder and this defect usually appears in high gloss black powder.
Try to change your supplier.

Ali Gomaa
- Cairo, Egypt

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Q. I have this problem that could be related to yours, but I like the speckled look. I found out by accident that the black powder seems to draw in airborne dust. Some dust [yellow] was coming off my oven and laying into the black piece I was working on. Other colors don't seem to do this. Is your shop spotless? Almost impossible for me to keep mine clean :-)

i do have another question for you. I know all about cure time, but how can I look at a finished piece and tell if it is cured or not cured right?

ronald d flahart
- lancaster pa. usa

A. Testing cure of a coated item is easy - find an area that will not be seen - take a small silver coin and draw it across the powder using enough pressure to cause damage to film. Look at the groove the coin made - if it is smooth then the powder is cured - if it is jagged then the powder is not cured. It is that simple.

Terry Hickling
Birmingham, United Kingdom

Q. From the above responses and other research I am convinced that the specks I'm seeing are the fluidizing post-ad aerosol. I have screened my gloss black powder through a very fine screen and got acceptable final results. I have adopted a new routine based on the fact that the additive is needed to prevent caking during storage and must be removed just before use. Is there any other less labor intensive possible solution?

Larry Johnson
Larry Johnson
- Key West, Florida

A. I am surprised as to why small black specks are so much of a problem. I have noticed them too. Fumed Silica [affil links], yes, resin flakes, yes, micro degassed bubbles, yes. So what? Usha, does it really matter when you look at the object from normal viewing distance. Or do you coat something really small and fancy? Did you try a matt/ dead matt black powder, I rarely see any specks in lower gloss powders.

Amit Amembal
- Mumbai, India

A. Dear Usha Patel,
In general , Reflection /gloss is too high in black shade than white shade.So defects are more visible in black glossy coating film than white or any other shade.

we feel problem of sparkle in black glossy powder coating may be due to poor dispersion of carbon black pigment during manufacturing.Carbon black need high torque % in extrusion process for complete dispersion.

Try other powder coating paint of different manufacturers.

Thanks and regards,

Jagmal Singh
- Gurgaon, Haryana, India

Q. We are experiencing white contamination sitting on top of black and other dark colors. One of the suspect is we changed our marker board (white powder and just thinking if we sprayed white powder which is polyster based.

After spraying white, it may be taken away in oven due to high velocity or left as ash on the hooks / racks and become air borne contamination and would stick to dark parts.

What is the possibility...? Still couldn't figure out where is it coming from...? Oven is clean.

Thanks for advice.

Kalemm Muhammad
- Holland Landing, ON, Canada
May 16, 2008

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