Corn cob has been the media of choice for several years. Crushed glass and glass beads are popular also, but they are very abrasive to windows, wires and trim. More costly also. Corn cob is very forgiving. I know that there are companies that use walnut shell but it needs to be a fairly fine grit since it too is a little more abrasive that corn cob. Any of them are okay as long as the person doing it has experience. If not blasted properly the marks will always be there unless sanded off.