I no longer keep mine sitting in front of the window, not that it seems to make any difference. I have never had any get hard and I have kept it there for a couple of years sometimes. It does sound like some of the other part got into your bottle. Doesn't take much!! Like David said, you never know how old a product really is unless it has a borne on date. I have had people bring me over filler to use on a plane I'm building for them and opened the little tub and it was hard. That's a product I always open the lid and take a feel before I buy. Epoxy????