We had quite cloudy weather for wuite long and sudden sun cause some burns. Especially harmed, apart from Euphorbia enopla fresh cuts in perlite/sand mix, was one monstrosa opuntioid I have, which I have also burnt in the past - this time it was even worse though. Photos forthcoming.

I find this of particular concern, because it seems sun changes can alone scar or even kill a plant! I sometimes become obsessed with giving my cacti as much sun as can, but as it seems not all like it as much. F.e. I think gymnos stall when in full sun, hey like it shadier - I wondered why one of my baldianums didn't progress a lot

My best bet is keep an eye on the weather and get up early and water them on hot days. We have a hellish sun here, I had been moving a small golden barrel from shade to sun slowly, he'd been out in the sun for only two days when we got a freak 40 degree day and he copped really nasty sunburn. Which shocked me actually, I always though they were quite hardy.
Move all of your pots out of the sun, especially if they're on cement or pavers, the roots will fry on the hot cement.
My partner (bless his long suffering heart) suggested we have a roll of shade cloth handy, or even attached to the fence to pull over the garden on really hot days.
My Dad generally opens the vents up on his tomato greenhouse and covers it in shadecloth, I am assuming cacti would benefit from this.

Contrary to what I always assumed, succulents can dislike really bright sun, I finally got some trigonas to grow really well by putting them in shade, so I suggest moving your euphorbias into shade, and some of our old crassulas seem to really suffer on hot days. Main thing is keeping the roots cool and direct sun off

yep alot of euphorbia types and gymnocalycium enjoy a bit of part shade/sun ...the sun here in west australia during summer is fierce so have alot of my succulents and seedlings and sensitive types under shade cloth ....still havent got that green house yet but one day ,one day

Thanks a lot.

Thing is I really like the idea to keep the hardiest plants like my columnars which love full sun, in full sun all year round, but I have made part shade spots too for pere/selini/gymnos and whatever else I find it prefers these spots. I suppose this change might protect them but not make them grow slower, I have even noticed that gymnos grow slower in full sun than in part shade...

Which do you find is the best programm, morning sun best, afternoon sun best? I think noon sun is preferably avoided if I can get morning/afternoon one...

Morning sun I think, afternoon is the scorching type, but our sun is horribly hotter than European sun. On any hot day I try to keep them covered until evening, as you just saw I had a cereus get sunburn, and they're really hardy. If you haven;t had any trouble with your columnars so far then I wouldn't worry about it

I've had scorching occur on a few Opuntias. It definitely seems that afternoon sun is the main villain.

I am more worried about leuchtenbergia, stenocacus and a opuntia cristata, not the columnars

Definitely keep an eye on the Opuntia cristata (is it a cristate microdasys?), as I find they're really twitchy when it comes to direct sun.

Ehm, my un-IDed cristata opuntia is dead I forgot to mention. After an intense burning and rainfall... I don't think is was microdasys.

You know for a hardy plant they manage to die a lot...