Concrete Coasters

• Clean the lids with soapy water using something that won’t scratch, like a 100% cotton cloth.
• Use a light mist of spray adhesive in the lid if you want to include decorative aggregate, crushed glass, or any kind of inlay.

Step 2 – Mix Concrete and Pro-Formula

• Depending on the size of your project, you may want to mix the concrete by hand.

Step 3 – Cast and Vibrate Concrete

• Make sure the mold is clean and free of debris.
• Press a small handful of concrete into the mold and pat it down to work out any air bubbles.
• Tapping the mold on the tabletop will further drive out any air.
• Remove the rocks that protrude from the top surface. If you don’t have an orbital polisher, grinding them down will be very time-consuming (nearly impossible).
• It is important that the top surface is flat and smooth to the edge of the lid. Smooth the top of the concrete by screeding it with a straight-edge or a trowel.
• Place the concrete on a level surface and cover it with plastic to cure for 4 days. The plastic will help keep the humidity inside; if the concrete dries out too quickly, it can cause small cracks.

Step 4 – Polish the Back Flat

• 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.
• Knock down any sharp edges with a diamond hand sanding pad or a coarse sharpening stone.

Step 5 – Demold

• You will break the coasters in half during demolding 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 demolding.
• 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.

Step 6 – Cast Finish vs. Ground

• The coaster on the left has a uniform cast finish, straight out of the plastic mold.
• The coaster on the right has been ground and polished to expose the aggregate in the concrete.

Step 7 – Seal the Concrete

• Concrete is vulnerable to etching and staining from anything acidic like lemon juice, vinegar, red wine, etc.
• Sealing isn’t necessary, but it will help protect the concrete from staining and keep the coasters looking nice for a long time.
• If your coasters don’t sit flat, you can cut thin cork backs or little cork feet and attach them with spray adhesive.