Film backs are rather precise. Casting and machining spec's would have taken into consideration the paint thicknesses, but this would be for paint contemporary to a 50+ year camera. You should be okay with typical rattle-can spray paints but I'd be cautious of using high-build modern chip-proof coatings. I'm especially thinking of the film holder gate-- particularly the base side toward the lens where the film holder rests. Too thick of a paint film build and you may start interfering with the focusing by moving the film plane. Similarly, too thick paint on the Ground Glass half of the back could affect focusing accuracy. Goes without saying that if you use wrinkle paint, use it only on the exterior parts of the back, not the measurement critical surfaces of the gate.
Also avoid excess paint in the grooves that receive the film holder ribs. (I once found a spot of corrosion and associated chip of lifting paint as the source of a light leak.)