Question
I've been raising cactus of all types, medium size, for years but with some inevitably in time they get what I call blisters of crackly light brown scabs.  My instinct is to use a sharp blade and cut this off since on one occasion I observed it spread and killed "my baby" whereas if I cut it out gently while small seems to do ok.  
Anyway, this summer we broke records in Chicago for heat and sun and 70% now have these scabs.  Your advise most gratefully acknowledge since I note they spread.
Thanks.

Answer
Marta,

About your rating of me. First of all I am a volunteer who has no formal training but has over 35 years of cactus collecting under my belt. What I do not appreciate is your throwing me under the bus because I did not know (based on your description), the exact cause of your plants marks.

It is people like you, who ask a non descript question, but expect a text book answer, that cause people like me to stop volunteering our precious time. I apologize for not giving you the answer you were looking for, but did the best that I could based on the information given. A piece of advice, ask a follow up question before you rate someone else as poorly as you rated me.

Sincerely,

Greg