Some times with the casting of the metal you use flux to help get the metal pure. and i found from the little parts i did sand cast. that if you use to much the metal will set like swiss cheese. The little pockets do form in the casting if the casting is cooled to quick or if it is under pure. But that stuff blademan is talking about works.

If you got a lot of pockets busting out you may want to try to use metal to metal or lab metal filler ..

You can also preheat the part to 130f for 15 minutes pull and shoot put back in the oven cure for 25 minutes at 400f That may help you