Step 8: Angle Grinder (optional)
After the edges have been smoothed you may notice that some of the edges look shiner than the flat surface of your opener. This dark coloration (aside from the layout fluid you applied earlier) is called mill scale. Mill scale happens when the steel is rolled flat and needs to be removed if you want a shiny finish.
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.