Ever noticed you can rub some wool on a piece of plastic and build up a static charge?
Also, you can powdercoat non-metallic parts, so you don't need the part to be metals in order to apply a powder finish, if the part is pre-heated in an autoclave, you can apply the powder finish without statically charged powder and a grounded component.
In fact, this process is used to powdercoat plastic parts (there are a couple types of plastic with melting temperature high enough to withstand the powder curing process)
Thanks for the clarification, it's good to know!