Yeah, that's one of the differences compared to polymer clays: you need to let it rest for a while before starting (15 mins if you want it to be more flexible, although it can still be a little sticky; 30 mins if you want it firmer and less sticky, better for small details). Then again, after an hour or two it starts to crack and tends to form those feathered edges when you're trying to sculpt with it. Water helps a little but you can't go overboard with it... that's when you should stop and just mix a new batch. The leftovers can be used for something that doesn't require detailing. If your Apoxie is old like my current batch - and I'm pretty sure it was old already when I got it - it starts to cure much faster. Mine is pretty much impossible to do any detailing with after an hour...