Cause of Porosity

What is the most common cause of porosity on a powder coated surface and how much porosity is considered acceptable?

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Q. What is the most common cause of porosity on a powder coated surface? How much porosity is considered acceptable? Is there a guide line for this measurement? Is there a mil thickness minimum to not cause this? What would be the cause for porosity on the surface, if we know the surface is clean and prepared to accept the coating? Under a 25× microscope, we can see small holes down to the surface. The coating seems to be gassing out on top of the film. It looks like a black speck but is actually a bubble with a hole in it. —M.T.

A. The most common cause of porosity in a powder coating film is trapped air or contamination on the substrate. A formed-metal surface, galvanized steel surface, cast substrate or any surface with existing metal porosity can cause out-gassing (release of trapped air) during the cure cycle. If the substrate is the cause of your problem, you will need to add a step to your process or work on a powder formula that has an out-gassing additive. Pre-heating may help to relieve some trapped air. Double coating may be necessary. Casting can be impregnated with a resin to fill up the porosity.

It is also possible to get some micro-porosity in a powder film that is not generated by the substrate, but it is unusual for it to go through to the substrate. The powder formula may have components that evolve from the film and leave some small depressions. If you have some minor holes at the surface but the appearance is acceptable, it may be all right. If you need good performance, such as corrosion resistance, the surface cannot have any holes through the coating to the substrate. Holes through to the substrate are more unusual and more troubling than some surface eruptions. That is more likely something related to substrate condition. You may want to blast some parts or clean them in some other way to ensure you do have the clean surface you think you do, or try one of the other methods mentioned (preheat, double coat).

The thickness of the coating also may be a factor. Some powder materials may be sensitive to films above 3 mils. Be sure you are applying the material within the manufacturers specified thickness tolerances on the Technical Data Sheet. Certain materials are more sensitive to thickness. You can do some experimentation with your powder to find out if this is the issue. Coat some panels at different thicknesses and look for the porosity. This will test the thickness and also show if the issue is substrate- or powder-related.

I am not aware of a method to measure porosity in the film. You can use a pin-hole detector to determine the severity and see if the holes truly go through to the substrate, but I do not know a way to quantify the results. If the porosity is in the film and related to the powder formula, it would be wise to consider a change to a different powder, if that is possible.

In general, I would recommend some trials to be sure you have the root cause of the problem. If it is substrate-related, you need to work on your preparation method. If it is powder-related, you need to work with the supplier to change the powder formula.

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