Metal finishing Q&As since 1989

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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?

RUPESH KUMAR
Powder coating application manager - Goa India
July 20, 2023

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⇩ Related postings, oldest first ⇩

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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
2006

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
- Key West, Florida
2006

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 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 (http://www.wovenwire.com/reference/particle-size.htm)shows a 7 mil particle size for 80 mesh and I'm wondering how that will help. Larry Johnson - Key West, Florida

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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
2006

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
- Key West, Florida

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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