black is best used on a sisal ... i work that til i get a nice clean dull shine . then i load a 6" sprial and brown till i get a nice shine going .. this step is to do the final cut and polish .. then i load another 6"sprial with white .... i use a bench mount and a dewalt grinder with a 6" on the bench i go up and down with the grinder i go left to right... are you cleaning the part good after the black?