My last comment / suggestion, which I'm sure overlaps w/ others' feedback above:
1. Use some type of speed controller if the grinder is limited to ON/OFF; you can even use a cheap light rheostat from Home Despot or what someone else mentioned from Harbor Fright. Regardless, you have to be able to slow the grinder speed to have some level of control
2. Use 3/8" OD sanding rolls to limit the amount of material being removed; get 60 or 80 grit, not the finer grits which tend to be more for surface finishing
3. Mark off where you think you need to remove material
4. Wear safety mask & goggles
5. Take your time --> sawzall