Posted By:
William BrumbachI have older sheets that I still use as well, probably about 20-years-old now. Well, not that 1985 is old, but, nevermind. None of mine are sticky or gloopy, that doesn't sound good.
Newer Ultra-Pro sheets will stretch over time if the fit is tight. I have only done this with thicker new cards however, so the risk with anything else is up to you regarding any possible corner damage by trying to stuff slighter larger cards into tight pockets.
Heating probably isn't a good idea because the sheets will wrinkle when they cool down. Having done more than my fair share of flea markets in the past I have seen the effects of the full spectrum of weather on plastic sheets of all makes and models and I highly recommend against any sort of heating or freezing of the sheets. Hot plastic smells funny.