I never heard of this and would be reluctant to even attempt it. I can't imagine it being a good thing.

In my experience, after overwintering indoors, cactus can be put right back into direct sun as long as it is not too warm yet, and there is good air circulation.

So my advice would be to find a time when for about a week where the temps stay on the cool side during the day, maybe in the 50's, and then they will gradually acclimate as the temps start to warm up as the springtime moves along.

The risk of sunburn occurs when there is heat that goes along with the sun. Air circulation helps as it dissipates the heat. In a greenhouse in can heat up rapidly with inadequate ventilation, so plants can get scorched in that type of situation. Outdoors conditions are more favorable.

I keep all my cactus in the garage all winter with no light. I put them back in my greenhouse usually around mid-February. It is not warm enough yet for sun to become dangerous.

My March-April the temps have warmed up considerably and the plants have acclimated perfectly well.

I searched around and could no longer find the product. I could have sworn I saw it at Yucca do or Living Stones Nursery. Soooooo anyway, the reason I was asking is that I had to move a small O. aciculata a few days ago and I turned it around exposing the part that had been shaded into the sun + it now gets much longer exposure. In summer I would have been careful but since its winter I didn't worry. Well, it burned and I have it screened now. (I used a single sheet of toilet paper on the pad which sticks to the glochids and I assume will dissolve when it finally rains here again) I couldn't believe it scorched. It made me remember back about the sunscreen spray stuff. Thanks for the advice. I may try that garage winter storage next year. I didn't know about the heat being the cause of the damage. We got into the 70's but the rays of the sun are so low. Live and learn.

I was googling and landed on a sight that talked about rescueing desert spoons in Arizona. They marked what side that went south and oriented the plant in the same manner because of fear of sunburning the north side. I found that issue of turning to be very interesting. I will now be careful of this.

Whether cacti can go out straight into the sun really depends on local conditions, and on the plant. Here in Minnesota, a significant number of mine burn easily if they get stuck outside in the spring while still dormant. I seem to spend all the way until late June shuffling plants around to get them ready. Most of the ones I have trouble with are shade cacti in habitat, which makes sense.

Xerophyte, could you put the plants into direct sun without the stage in your greenhouse? Am I correct the greenhouse is unheated? If I had an unheated greenhouse and wanted it to be safely above freezing, I couldn't put plants in it until April.

I think things might depend on the temperature pattern here in the spring. From when I lived in Boston, I recall that spring kind of came gradually in the northeast. Here in the upper midwest it tends to stay consistently cold up until a fairly late date and then warm way up quickly. I suspect this is harder for plants to get acclimated to.

My greenhouse has a heater on a thermostat that shuts off when it hits 45F, so it doesn't really provide warmth - it is just to keep it from getting too chilly overnight. My cacti are sitting on dark shelves in my cold garage right now.

I place my plants in the greenhouse usually around late February. I don't use any shade cloth. But I do have 3 big fans blasting at full power to keep the air circulating very well. 2-3 weeks in those conditions, and the plants are safe from burning going forward.

The only time I've had problems is during a sudden heat wave where temps may reach the 80's in the greenhouse. A shade-cloth would be needed at that point.

Undoubtedly, the powerful fans negate the detrimental effects of strong sun when it warms up in the greenhouse.

Rebutia seem to be particularly prone to develop some bleaching in the sun.