Flap disks (ususally used on 4, 4 1/2, and 7 inch angle grinders) and flap wheels (usually used on bench and die grinders, and drills) were designed to grind down and finish welds ... in ONE step ... and leave them ready to go in about 25% of the time that you would spend with hard grinding wheels.  They are VERY aggressive!  They are also one of the best items for deburring, and for removing excess fiberglass, before you put the hardcoat on.  They seem to load at about the same rate as they wear out, so they just keep working.  THIS ITEM IS FOR AN INDIVIDUAL 4" FLAP DISK.  YOU MUST INDICATE DESIRED GRIT - 36, 40, 60, 80, 120, 180, 240.    CALL 800-631-2149 WITH ANY QUESTIONS.