These are the first castings I ever did (no idea why I chose something so complicated to learn on). The ones with holes in them I think are a combination of inadequate venting to allow air to escape when pouring in the aluminium and I was also pouring the aluminium too slowly. So with better vents and faster pour some eventually came out quite well. I was using home made green sand and how much water was in it made a big difference to the finish as well.