Step 8: Angle grinder (optional)
To remove the mill scale we'll use an angle grinder with a flap wheel. Clamp the bottle opener flat onto your work surface and use the grinding wheel to remove the scale. This will create a shower of sparks, so use caution. You'll only be able to grind one half of the surface at once, so unclamp your opener and grind the other side, then flip it over and repeat.