mine always ends up a tad tacky as well but as soon as I hit sandpaper to it, it mysteriously disappears It could be due to humidity or as Contec mentioned not enough hardener.
The tacky problem is moisture that's causing that problem, as for trekman's problem not enough hardener. I would usually mix a batch on a piece of cardboard and the pile of bondo on the cardboard I put a strip across the top from one end to the other and it's usually the perfect amount.