I've had a total of 10 greys. one of my current girls Emerald is really my only permanently bald butted girl. Tanzanite has very thin fur over her butt, but it's there. Opal has a small bald patch here and there on her butt. (she never raced or even went into the race kennels...so it has to be genetic). But I can't really say she's got a bald but. Just a bald patch, the size of a silve dollar.
However, Topaz, Brindle, Pearl, Diamond, Onyx and Crystal all had the thickest fur. I miss that. Not a bald spot among them. I loved running my fingers thru their fur. It was gorgeous for all 6 of them.