i need to fit a 5/8" square bar into the square hole in a corbin  brand seat.  i measured the bar today and its  0.621" . i painted it with flat back paint and it still fits in the hole but a little bit tighter. my question is how thick is powdercoating?

It depends on the type of powder...

Thermoplastic resins need to be classified as.
PVC
Nylon
Polyester
polyethylene
polypropylene

There are two application methods for thermoplastic resins.
Fluidized bed technique ...... results in coating thicknesses between 5-12mils.
Electorstatic spray ....... results in coating thicknesses between 3-5 mils.


Thermosetting resin yields a coating thickness of 1-2 mils.

thanks for that tussuck, its gonna be too thick

Black chrome will work on the back rest post. If you want to use powdercoat just have it done on the top half of the post just above where it go's into the leather cover.