Powdercoat gas bubbles on zinc plate steel

Letter 2026

Powdercoat gas bubbles on zinc plate steel

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we have a finishing problem on zinc plated steel wrought iron work. the work is zinc plated (not chromated) then finished in polyester powdercoat.

On the areas where welds have been ground flush, the powdercoat finish gets gas bubble holes (porous looking). We have tried varying the pickling procedure, degassing before powdercoating, different grinding disk compounds.
Zinc phosphate pretreatment is used.

The problem can be virtually eliminated by finishing the welds using an 80 grit (fine finish) disk but the extra cost in grinding smooth with 80 grit needs to be eliminated. we prefer to grind with a 24 grit (coarse) grinding disk as it costs less in time and disks and the 24grit finish is acceptable to the end users once it is powdercoated.

Some powdercoat applicators can powdercoat over the coarse finish and not get bubbling,they do not pretreat in any way except preheat the work. however the adhesion is poor. (maybe this gives a clue to the problem)

John Bransby
- Sydney, NSW Australia

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John:

Are you sure you're getting all the grinding swarf off the surface of the weldments?

This may be off the wall, but I have seen some unusual things. When you grind steel, you generate alot of very small particles. Now small particles of a particular size have an electrostatic attraction for the steel surface. If you do not have spray pressure or in the case of immersion ultrasonic waves with enough energy to overcome the electrostatic attraction of the fines, they may not be removed from the surface of the weldment.

What could then happen is that during the application of the powder coat the fines stand up (They are iron, by the way and are magnetic) and get trapped by the powder and cause the defect you are describing.

Again that may sound off the wall, but I will also say that the way to figure out what the defect is is fairly simple. You need only two things: a microscope and a razor blade.

Here's how:

View the defect from above to determin if anything protruding. Make notes.
Slice a little of the paint away from the defect and view again. Make Notes.
Repeat #2 until you can see the substrate.

You'll find either something or nothing. Nothing generally means solvent pop or degassing of some sort. If you find something, then the hunt is on to determine where it came from. Here's a quasi-diagram of the process:

Bubble Bubble Bubble Paint Film Paint Film Paint Film Paint Film Slice 1 Slice 1 Slice 1 Slice 1 Slice 1 Slice 1 Slice 1 Slice 2 Slice 2 Slice 2 Defect Slice 2 Slice 2 Slice 2 Slice Slice 3 Slice 3 Defect Defect Defect Slice 3 Slice 3 Slice 3 SubstrateSubstrateSubstrateSubstrateSubstrateSubstrateSubstrate

Good Luck,

Craig Burkart
- Naperville, Illinois

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It is possible that when using coarse grinding discs, some of the abrasive becomes embedded in the steel surface. This may not occur with finer discs. What is the coating adhesion like? If it is satisfactory, then try "casting" grade powders as these are more forgiving to popping.

Another possibility is that the zinc phosphate deposited on the coarse ground surfaces may itself be coarse and crumbly (does the phosphated coating sparkle in the light?). If this occurs then the coating will exhibit popping and roughness.

Mario Pennisi
Powder Coater Painter - Brisbane, Queensland, Australia

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An update on progress so far . we have changed to a powdercoater that uses zinc phosphate pretreatment. most powdercoaters seem to use iron phosphate we found. With zinc phosphate pretreatment and a powder that allows gas to pop through we have eliminated the problem so far. It could be either the different pretreatment or the different powder. anyway we now know that there is an option to using very fine grade sanding disks.

Thanks for the help.

John Bransby
- Sydney, NSW Australia

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