Should I use a Flap Disc or Grinding Disc?

Grinding discs fare best for heavy stock removal and roughing, while flap discs are best for light material removal and for smoother nicer finishes. Flap discs also have a little bit of “give” so they’ll leave more of a molded or rounded surface when rounding off edges. On the other hand, when it comes to removing rust or other heavy coating, flap disks will fill up and stop working nearly as well, while a grinding disc will keep going.