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topic 60844

Bad powder coating on steel: Poor wet out surface defects

September 23, 2018

Q. Hello, we are a company working on steel. We are facing many lack of paint issues especially on the edges of the parts with some RAL composed of epoxy-polyester and polyester. At first I thought of Faraday cage effect, but after checking I realized that we use the tribo system, so this effect is not supposed to occur.

Our pre-treatment is a conversion one based on Zirconium nanocoating. Rinse waters have been checked and rust appearance has been mastered, but issues still appear after powder coating application.

Do you have an idea where these issues may come from?

I attach some pictures of defects:

60844-1a   60844-1aZoom   60844-1b   60844-1bZoom

60844-1c   60844-1d   60844-1dZoom

Thank you

Nadia Bankole
- Cairo, Egypt


September 24, 2018

High Performance Powder Coating
from Abe Books
or

A. This is almost certainly the result of spraying powder onto a wet substrate and then baking it out.
Hope this helps.
Regards,
Bill

William Doherty
Trainer - Salamander Bay, Australia


September 24, 2018

Q. Thank you for your reply, actually parts are well dried (oven) before coating application. This point has been already checked thoroughly.

Regards

Nadia Bankole
- Cairo, Egypt


September 2018

A. Hi Nadia. If they are not wet, then they are certainly dirty or inactive. This is not a powder quality or curing problem ... it's a substrate which either due to not being dry or not being clean or not being properly acid activated or having a deficient pretreatment is not accepting the powder properly.

Powder Coating Complete
from Abe Books
or

I suggest sandblasting one sample before sending it through you pretreatment line. I am confident that it will come out fine and will remove your worries about Faraday cages, or other doubts about your powder and its application.

You might also try a sample straight from sandblasting to powder coating and, although it will probably won't prove satisfactory for the long run, I doubt that it will peel and pop at edges and corners like this.

Regards,

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


September 25, 2018

Q. Hi Ted, in fact the pretreatment encompasses an acid cleaning, the conversion step. Then, there is the coating process. I made a wettability test just after the cleaning process and it was successful (the dropping water test).
We don't use sandblasting before the chemical process but rather a grinding step as a mechanical activation. Maybe I could explore a sandblasting approach.
What I don't get is that this problem only appears with some specific RAL. On some RAL we don't even face this trouble.

It's not a Faraday cage effect as we use a tribo system and not a corona one

Thank you for your help.

Nadia Bankole
- Cairo, Egypt


September 2018

Powder Coating Systems
from Abe Books
or

thumbs up sign  Hi again. Well, if you know it's the specific powder then it's the specific powder and there's no more to be said. But it does strike me as very surprising based both on the look of the defects and the fact that the color of a powder coating really shouldn't radically affect your ability to apply it.

I'm always happy to learn something knew, but I wish my confidence level in what I just learned was a bit higher :-)

Regards,

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



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