Q: What are the possible reasons for micro-pinholes in powder coating?
A: This is a very common but oftentimes perplexing problem with powder coatings. And there are many possible causes for the incidence of pinholes. Let’s run down the list.
1. Hot-dipped galvanized steel (HDG)
HDG is notorious for causing pinholes in a powder coating finish. Some say it’s due to the evolution of hydrogen, while others think it could be from entrapped air/moisture.
Regardless, it is always wise to preheat galvanized steel prior to applying a powder coating. You can let it cool to almost room temperature, but you better get the powder on quickly or the pinholes will reappear.
2. Volatiles of cure
TGIC-free polyesters (aka hydroxy-alkyl amide or Primid™) will exhibit pinholes if applied too thick. Films over 100 microns typically show this defect. Ditto for most polyurethane powders. They emit a blocking agent (typically e-caprolactam), which causes pinholes at thick films.
3. Moisture
If the powder container has been left open in a humid environment, the coating may absorb moisture that can cause pinholes during film formation in your oven. Moist substrate can cause the same problem. So keep your powder dry and make sure your substrate is completely dry before applying the powder.
4. Bad or old powder
If your powder is over two years old, it may have picked up moisture even if the container appears to be sealed. Try fresh powder to see if the problem disappears.
5. Contamination
Cross-contamination between dissimilar powders could be the culprit. Keep your powders separate and thoroughly clean your application equipment when switching powders. Defects caused by cross-contamination can range from lower gloss to pinholes or even craters.