personally I wouldnt worry about the scaling too much, when Ive had black rot on my cacti, I just keep them apart from the others a little bit in the vague hope that it wont spread, and they have all come good again, but with a little scarring. feeding your cacti seaweed fertilizer is well and good but they will really respond well to more nitrogen, so maybe if you use blood and bone or worm juice or something along those lines you will maybe see more results. I like to leave my cacti to their own devices and they look awesome with only the original cut marks, and they pup where they want when they want. Thats my two cents!