Molak Stucco filler

I bought some model filler a while ago that says Molak Stucco on the yellow and blue tube after some LHS dude said it was good stuff. The trouble is I haven't found it to be so good because it has a habit of sometimes just falling off again once it is dry when I begin to sand it. It also has a load of tiny bubbles in it once I sand it level.

Has anyone else had this problem with this brand of filler or any other brand of filler from a tube for that matter, and does anyone have any tips that might prevent the problems?

TIA, Matt.