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[title] Watch for and remove air bubbles
[* black] Because each penny has a raised edge, it forms an air pocket underneath. When the heavy epoxy liquid is poured on top, it forces the air out. The epoxy is engineered to expel air bubbles naturally, but particularly as you get closer to the epoxy setting up, they may not break by themselves.
[* black] The instructions recommend the use of a propane torch. They suggest that it's the CO2 from the flame, not the flame itself that encourages the bubbles to pop. I think it's also heating the air in the bubble which causes it to expand rapidly and pushes it toward the surface.
[* black] Run the torch quickly across the surface. Do not stay too long in one place. The epoxy is not flammable, but it will burn and discolor if you stay in one place too long.
[* black] We also had success using the corner of our acrylic spreader tools, as well as the point of drywall screws to pop the bubbles.
[* black] No matter how much time you spend at this, some bubbles will end up hardening into the surface. You can use a drill bit to remove the epoxy over the bubble and smooth the edges so the epoxy can run into it on the second coat. Our second coat had no air bubble problem.