I've had this pair of fairy castle cacti for several years now, and they've done fairly well. However, I noticed today that they have developed brown patches over most of their stems and a few areas have a sort of crackled look. I can't say exactly how long it took for this to develop, but I didn't notice it on the last watering (6-8 weeks ago).
in some areas it looks "built up", like it could be chipped off (though I haven't tried). There are also a few very dark spots that almost look burnt, except they are on the side facing away from the window.
Any ideas on what is wrong with them, and what I can do about it (natural methods only please - I don't use commercial gardening products)?