Why did my cured epoxy top crack months later, and can it be repaired?

Once UltraClear Epoxy has fully cured after 3 days, the chemical reaction is complete and it becomes an inert plastic that won't keep reacting or break down on its own. When a crack appears months later, the most likely cause is the substrate under the epoxy moving, not the epoxy itself.

Wood is the usual reason. If the underside wasn't sealed, the wood absorbs moisture and expands and contracts with temperature changes, which stresses the epoxy above and can crack it, even if the surface was previously smooth. Outdoor projects are especially exposed to this if the wood isn't fully sealed on all sides. Hairline cracks after a hot day in direct sun are very uncommon with our epoxy; when they occur, the wood's moisture content, differing expansion between wood and epoxy, or conditions during the original cure are more likely to blame.

To repair it, sand off the damaged epoxy, finishing with 220 to 320 grit, wipe away all dust with acetone or denatured alcohol, and pour a fresh flood coat of UltraClear Bar & Table Top Epoxy; it fills the sanding scratches and cures back to a glass-like finish. Before re-pouring, make sure the wood is completely dry and sealed on all sides, for example with multiple coats of oil-based polyurethane (or a seal coat of epoxy) on the underside, and keep the room at least 75°F for the full 3-day cure. For pieces kept in the sun, add a UV-protective polyurethane over the cured epoxy.