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Powder coating is bubbling up off the surface.

Hi I built my own steel railings and had them powder coated. My plumber immediately damaged them by scratching chipping and burning them with his torch. My powder coater said he could sand these problem spots and re-coat the damaged areas. All the railings bubbled when he reintroduced them into the oven. They are ruined and need to be sanded clean and completely re-done. He tells me he has never seen this in 20 years and that the problem has to be paint on the surface. I think something went wrong with his process and he is trying not to be responsible. Have you ever seen this before and what would cause the powder coating to bubble off the surface like that? I've also noticed a few rails at home that have the same kind of bubbling that suggests a defect in the first application.

Rena E Skubish
customer - Westfield, N.J., U.S.A.
January 2, 2008

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First of two simultaneous responses -- January 10, 2008

Its a popular misconception that powdercoating as a single coating on mild steel used in ourdoor applications is a good corrosion protection. Its not.

Either a multi-coat system where the first coating is a zinc rich primer, or even better hot dip galvanizing followed by powdercoating is a better spec. This latter will last for over 25 years.
If your plumber or anyone else scratches them, they won't rust bubble or chip, and if you really want to repair a scratch on such a system, a small dab of paint to match the colour over the scratch can be done in-situ.

The bubbling you saw suggests several possibilities.
Could be mismatch between the two coatings (1st and 2nd).

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

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Second of two simultaneous responses --

Dear Rena, We have supplied powder for over 25 years and I have never come across a powder bubbling. Powder can blister when applied over aluminium casting and some time over galvanizing. What is confusing is that you speak about powder and then suggest the "paint on the surface". Paint would suggest a liquid coating and this would certainly "bubble" when introduced to heat. Could you check that your railings were "powder coated" and if they are, submit a photo to this forum so we all may see the defect you describe, then we may be able to assist you.

Terry Hickling
Birmingham, United Kingdom
January 10, 2008

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Dear all,
This very question has been discussed about 20 times on other forums. Nae was given very good advice from applicators and mf -- very similar to that which Rena received here. Please do not go from forum to forum seeking an answer that suits a possible legal matter.

Terry Hickling
Birmingham, United Kingdom
January 14, 2008

Terry, I'm busy enough here that I don't visit other forums too much, and don't know what you're alluding to. We have more inquiries than we can print, and don't knowingly print inquiries that have been printed on multiple forums because the inquirer tends to forget all the places they posted and then doesn't come back to say thanks or, more importantly, reply to the followup questions when they are asked.

Rena, the most important reason not to post inquiries in public forums if you're involved in a legal matter is because your attorney is likely to strangle you :-)

If the opposing side can show you seeking opinions of those knowledgable in the art and your position being rebuffed it can be damning. Your attorney will want to handle expert opinion as "work product", etc., so unfavorable opinions can be kept out of the record. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
January 14, 2008

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Dear Ted,
Although in this particular instance, it has not reached the point of attorneys, part of my input is to warn readers, if you are having a problem that you think has been caused by neglect of a second person, then contact a suitable Consultant. Take his advice and do not go from forum to forum seeking an answer, as the answers will vary from blaming you, the coater, the weather and God. In addition, as you rightly point out, you may be priming the opposition to various options why things may have gone wrong.

Why does powder bubble on baking after applying a second coat? I would suggest that the coater when attempting remedial work on the steel railings applied the second coat of powder to a cold substrate, he also used his normal Kv he uses on day-to-day basis. By doing this, he was attempting to coat a steel substrate that was electrically insulated by the first coat of applied powder. By not reducing the Kv back-ionisation (did I spell that right) occurred resulting in holes in and partial cure of the powder he is attempting to apply. No defects would be apparent until a full cure of the applied film has taken place. The defect would appear as small bubbles and the cured film may appear to be cheese like to scratching.
The rusting defect is another matter and requires investigation as to whether and what type of pre-treatment was carried out - for outside situations a zinc phosphate is recommended. If the first coat of powder were susceptible to rust creep, the defective second coat would not be able to prevent weather damage. Therefore, re-coating powder requires, lowering of the Kv and a hot substrate to allow the powder to melt onto the already fully cured first coat.
Terry

Terry Hickling
Birmingham, United Kingdom
January 17, 2008

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You cannot repaint some epoxies without completely sanding or removing the first coating of paint. The second coating will bubble up peel right off the first coat. My first thought would be that he used an epoxy and didn't properly prep before applying the second coating of powder. The paint bubbling on the first coating could suggest that he did some rework and didn't prep it correctly then either.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
January 17, 2008