How fast are you cooling them, jhepp? How's your workroom as far as temp, humidity, and air flow? For a while I was solving the sinkhole problem by keeping my workroom temp above 77*F, but then that didn't work any more, so I'm wondering if it's a humidity thing as well (as the humidity here is much higher during the summer). The later at night I pour them, the less likely they are to have a sinkhole, as well. (I don't know how much help that is, though. ) One other thing I noticed is that giving them a few stirs after the pour when they are <100 seems to have an effect. I noticed this after experimenting with some marbling (art for art's sake ). That's probably more trouble than heat gunning, though, really.