Can anybody tell me what this flakey tan coloured crust is on my cacti? It starts at the bottom and goes up, peals fairly easily but comes back. I've lost some but others have survived for years with it. I don't think I'm over watering and they are dry from Oct- late April early May. I try to water from below so water doesn't stay too long around the plant. Any suggestions oh how (if possible) to get rid of it and prevent it would help too.

Kinda looks like sun or cold damage...is that a possibility?

Brian,
Wish I knew. I've got the same thing on my Euphorbia Lactea. It is over 4 ft. tall. I was told it was sun burn. The stem turns to bark. Very hard. You said you can peal it off your cactus? I know I can't even start to peal it off mine. You might have some thing different. However, I do have some Euphorbia Lactea White Ghost that is almost all white and has stayed in full direct sun with out any signs of this bark. Also I have E. lactea Cresta that is also into direct sun all day and also doesn't show any signs of this bark. I am beginning to think that this is not sun burn but maybe some thing, I don't have a clue, else. Hope some more knowable people help us out.
Stush

Dunno. Cactus do like direct water they just dont want the roots to sit in soggy dirt, watering from below might have left them dry and dead on top. Also some of those cactus look like they have sun damage, when a normally green cacti starts turning purple or pink it usually means they got more sun than they were adjusted too. However the tips on some look green and fresh so maybe it was just a short period of exposure?

I'd say give them a good water with some liquid fertilizer, if your worried about to much water in the roots you can switch to terra cotta planters, a cactus/palm soil, and or add lots of pearlite till it rapidly. I doubt it will fix the damaged sections but at least your cacti's new growth will look good.

I think that is what they call corking. It usually occurs from the base and may also manifest in other parts. It happens naturally to cacti & euphorbia as they grow older but sometimes premature corking is also induced by other factors, like too rich soil or too dry air etc. etc. To me as long as it is not mushy and oozing, it should be okay. May be a bit unsightly as the brown color stays and does not return to green.

Thanks folks. Yes the one in the bottom right of the third photo definitely got too much sun too fast when I put them out this spring but it is looking better now. I think I will change all my pots to clay and fresh soil and see what happens. When is the best time to fertilize? Just in the spring or throughout the summer? Is it too late now? our nights are starting to get fairly cool and soon they will have to be brought in for the winter.

Most cacti go dormant when it is heading to winter. I would delay repotting to spring/summer. The plants are heading to dormancy anyways, so what will count more for now is decrease watering to none during fall and winter, and getting enough light & warmth while sheltered indoors.