Thanks. I just wanted to make sure I didn't give him the wrong treatment.Shell rot occurs because the shell is continually subjected to moisture, urine, wet poop. Desert tortoises are NOT prone to shell rot. Besides that, shell rot usually shows up first on the plastron.
I've taken in rescued desert tortoises in the past with this dry, white shell. I don't know what causes it, but my guess is it's some sort of dryness. What I did for them was to wash them with baby shampoo and scrub the shell during the sudsing with a soft bristle brush until it was as clean as can be. Then I dried the shell. When it was totally dry, I applied a thin coat of cold pressed coconut oil, allowed that to absorb for 15 minutes, then polished it all off with an absorbent, soft towell. I did this once a week until I noticed a difference.