When using our casting epoxies; MirrorCast and RiverCast, it is crucial to keep it in its mold until it is fully cured (7 days). While the epoxy may seem hard after a few days, it will not reach its full strength until it has fully cured. Having the epoxy supported in the mold while it develops its full strength is important as any stress or strain put on it too early (even its own weight) can lead to warpage or sagging. Also, many casting projects will eventually be machined, the epoxy will only reach its optimal properties for machining once it is fully cured. Finally, epoxies typically shrink on the order of 2-5% by volume. Waiting until the epoxy has fully cured will ensure that the volume is stable for machining and finishing.