Corking due to overwatering?

Could be a lot of things that end up damaging the epidermis of a cactus. Mites will do it. Sometimes a plant has a propensity to do this and the result is called corking. It won't hurt to examine it with a magnifying glass to see if there are tiny webs among the spines. Unfortunately soap has little effect on mites, don't know about Neem Oil.

I get a similar defacing thing on my plants and ascribe it to humid conditions we have in the summer. As long as it doesn't cover the whole plant or affect the apex, I generally keep it under control by keeping the plants dry.

If you do find mites, then you need to treat them real quick since they can spread and cause severe damage and eventual death.

It may be Corking. There has been so much raining this year in Florida. Is there any special soil that I can use in heavy rain environments like Florida.

I was thinking of repotting the cactus and placing it in pure sand. Could this help?