Step 3: Grinding + De-Molding

Picture of Grinding + De-Molding
Grinding with an orbital polisher:
  • While the concrete is still in the plastic lid, grind the top flat with an orbital polisher. Keeping it in the lid will give you an even edge to grind down to.
  • Un-ground concrete will have a uniform color without any exposed aggregate.
Grinding without an orbital polisher:
  • Concrete is extremely difficult to grind without the right diamond polishing tools. Even with a coarse diamond hand pad, it will take a very long time to flatten a 4" disc. This is why it's so important to smooth the back surface flat while the concrete is still work-able.
  • Use a lapping wheel with diamond discs.
  • Try a coarse sharpening stone.
  • Try a coarse wet/dry sandpaper (45 or 60 grit).
  • You will break the coasters in half during de-molding if you haven't waited long enough for them to cure (3-5 days).
  • Pry the plastic lid off by pushing down with a flat screwdriver, working your way around the lid.
  • Take your time and be careful not to scratch or chip the concrete during de-molding.
  • If the concrete is really stuck, try blowing down between the edge of the plastic lid and the concrete with compressed air.
  • If the concrete is still stuck, the last resort is to break away the plastic mold with wire cutters or scissors.
  • These lids can be re-used dozens of times, just clean up the concrete residue with warm soapy water or denatured alcohol.
Sanding the edges:
  • Knock down any sharp edges with a diamond hand sanding pad or a coarse sharpening stone.