Glass beading / crushed glass, cost more, will generate more heat, and you may end up warping your panels if your not careful,......What I prefer to do is use "medium cut" Black beauty,....blast the panel on an angle, don't concentrate too long, move along allow any heat to disapait, then go back and hit any area you feel needs to be addressed,.....Black Beauty leaves a slightly rough texture to he metal, but I feel it helps promote a good adhesion for a coat of vari-prime, or epoxy coat rust prevenitive base, which you should do immediately after blasting/blowing the car down