Zinc problems

Where I work we are experiencing a lot of problems with the zinc castings we are receiving from another company. While grinding these parts we get pitting along the areas where gate or flashing is on the raw material. We have tried fine grinding to avoid opening up more when we autopolish the pieces and we have tried coarser grinding to go past the problem. We haven't had any luck with either technique. The pitting is present after belting so when we buff the parts it gets worse. Does anyone have any ideas to avoid pitting or removal of the pitting.

Short and sweet answer, get a new supplier! No amount of grinding or material removal can fill voids or make the part look better. This is a casting problem not mechanical finishing.