Sheet parts arrive with cutting oil, laser oxide on cut edges and handling marks — all of it has to come off the surface before coating.
Bent parts need attention at the radii; the film thins on the outside of a bend and can build up on the inside.
Large flat panels show every flaw, so gloss level should be chosen deliberately: matte and textured surfaces hide small irregularities far better than gloss.
Perforated and folded parts are hung so the powder reaches inside the folds and the holes don't fill in.