YOu should only foliar feed at night when the stomatta are open.
We have found that this dramtically increases the bitterness of the plant. ALkaloids are bitter...Hmmm ya see where this is going?
Yea, calcium helps them protect themselves from the sun.
Add 2 tbs damp rid to 5 gallons of water and a tiny amout of chleated trace minerals, just a dash. directly foliar feed a few hours after dark. I saw a dramatic growth response in a few days.
The brown pups turned dark green in a week.
Just be sure they get lots of fresh air to evap any standing water on the cactus surface nooks and crevices, to prevent rot. Never foliar feed in the daytime when exposed to direct sunlight, it will burn the flesh, not to mention its pointless when the stomatta arent open.


Here is the advice I was given by a cactus expert at the nook.
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You need to foliar feed the cactus with a weak calcium chloride (damprid) or crushed Cal-Mag pills (from vitamin section at grocery store) solution every other night until they start to perk up. Use about 2 tablespoons of damprid or crushed pills in 5 gallons of clean water. Pour this gently over the cactus flesh at night. BTW, you can feed your other plants with fertilizer this way, too... it even helps to get nutrients to un-rooted specimens. 

The calcium will help them to resist the UV, and magnesium (if using pills) helps the cactus to utilize the calcium better... though they probably get plenty of magnesium from the soil.

When repotting from bare roots, you should never water for about the first week or so... the small amount of damage to the roots needs time to heal and scab up.

For now, place a thin see-through cloth over the pete. Leave it on there until you see them start to green up. Take it off, then, but leave her in the shady spot for another week. Then move to partial sun (evening sun, preferably) for a week. Then move her out into full sun. You may see some purple discoloration, but it doesn't hurt a thing... brown is definitely bad, though.

I have seen cacti recover from brown, however. Recovery is slow on their own roots, but I think a graft will do much better.

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