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JoannOK. I'm about ready to sacrifice a very low end card or two to try this UV thing out.
This is what I think: UV protection, whether natural or via additive, will help prevent fading although not completely bar it forever. I've found this to be true even in hard hats. I will occasionally put a few on the roof of the shop to see how they respond with and without the UV additive (they are not PC, so we use additive). Those without will start noticeably fading within about 3-4 days, and fade almost completely within a month or two on the roof. Those with protectant will maintain near-normal color for months, but if you leave them up there long enough you will start to notice slight degradation. So no, UV protectants aren't bullet proof forever.
As to cards, if I had UV protectant storage (even if toploaders could have it - not just slabs) I would look at it as providing protection from fading in incidental and normal daylight, but not if I leave them sitting out in the sun for weeks - or even days - on end. So it would help keep them nice in normal use, display, showing off, etc, but not if I'm a moron and leave them in the back yard for a week.
We have a UV accelerator in the lab. It's kind of like a tanning bed for flat panels of material. I'm going to take a few shiny cards and put them in there. I'll put one in what's left of a PSA slab, one in a regular toploader, and one between two pieces of clear 4x5 Polycarb safety plate. The UV chamber accelerates the effect of daylight. I'll see what happens and how much UV protectant can really help.
Because as to the normal kitchen counter, desk top, etc incidental daylight exposure, UV protectant slabs or toploaders would be awesome.
Joann