Tagger, you make some very good observations; However, I don't think your plugs spend enough time in the sun for UVL to have an adverse effect.
When I was molding polyurethane (MDI) automotive facia we would slow down the cure to prevent the article from curling. We wanted the same rate of cure on the the core side as we had against the warmer cavity side in mold position. We added a retarder, it was known by it's buzz word as DMEA. It was called Dimethylethanolomine (I think??). While we achieved out intended goal we still had UVL problems. Adding antioxidants would cause other "off ratio" problems so we cured that problem using a better prime coat at paint.
If you ever see a dented urethane bumper that has paint failure and the substraight has turned yellow, that's the UVL working on the Polyurethane.
Those free radicals are funny little bastages and they are taken back by the color gold. Besure to put a cover on the contailers with your nose, belly and tail grommets in it. Those little buggers steal them at night and hide them like a squirrel hides his nuts
