I powder coated for Barley Chalu, and UK Fire and Colour cote for years.
You just face towards you the part to be coated, then just wipe off any over spill.
I did all mannner of things(on the side), mostly m/bike frames, but car parts a lot as well, I always washed in chemical baths first all the bits to be done.
As said any surface with ANY residue of ANY type on it will stop it adhering correctly, and it will fall/flake off
tabetha