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Smut on powder coated parts after curing

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Q. I have a problem with high gloss black. It usually happens on thick parts, die cast iron and die cast aluminum when I apply high gloss black powder on parts.

After curing there is some kind of grey smut all over the part. It can be wiped off with clean cloths or polishing machine. But sometimes I need to do a very hard wipe to get this smut off. Result is less gloss, less shine after wiping.

I need help to fix this problem please. I've been searching answer for a while but seems like no one has this problem.
The result came up as photo attached.

My coating procedure is:
- degrease
- Strip old color by chemical
- Rinse by DI. Water
- Dry by air blow gun
- Sandblasting
- Rinse by Di. Water
- Dip in Zirconium bath
- Dry by air for 12 hours+
- Heat up in gas oven before coating at 120 °C for 15 minutes
- Wait for temperature decrease
- Apply powder color
- Bake at 200 °C for 15 minutes