I have used stop loss bags for a long time now.  Someone on here suggested them a few years boack....maybe even on LJ's.  It might have been Devin, as I recall.  They are heavy, I doubt they are UV resistant. I store mine out of the direct sun in a shallow tote, with no lid on a shelf.  they are a little floppy, but never really had an issue....it is a two person job as outlined by Rich to fill them.  I have 8 or 10 in this tote.  It is heavy so, I slide it out and grab what I need and slide it back in whole never taking the bulk of the weight off the shelf.  I have never lost any of what is in them to age or darkening.  I have used them for many kinds  of Poly, and varnishes, even some  paint.  I have never used them with any kind of oil, I doubt anything oil based would need it.  Considered Watco, but had some Watco that was ten years old in it's can and worked just fine.  I originally stored some water based Poly in the stop loss, I had tinted and thinned from 5 years ago I bet, and it sprayed like the day I got it.  I have gotten them a number of places and they are a little pricey but they do appear in the woodcraft flyer a lot on sale.  While they are weird, more like a thick bladder than anything else, I think if you use them in the intended manner and recognize them for what they are you will really like them.

Mike