You can't powdercoat plastic as far as i'm aware
I didn't think it was possible, but google says it is.
"Yes, it is possible to powder coat plastic.
There are three routes:
1. Use a very high temperature plastic (nylons, polyamides) with standard thermoset powders. May require a prep coat or pre-heating.
2. Use a variety of lower temperature plastics with a low temperature thermoset powder.
3. Use a variety of lower temperature plastics with a UV cure powder."