Can I flip my table over to finish the underside after the 72-hour cure?

Plan the underside first. If you want epoxy on the bottom of a slab or table, apply and cure that side before you pour the top. Once the top is poured, flipping it puts the finish at risk.

At 72 hours the epoxy is cured enough for light use, but it is still freshly cured and keeps hardening for months afterward. Turning a large, heavy top face down onto supports such as 2x4s and cardboard can leave impressions in the new surface. If you need to sand a drip line on the underside, do it without flipping the table if there is any way to reach it.

Small pieces such as resin clocks that have to be turned over to knock off drips or mount hardware can be flipped carefully. Lay the piece on something smooth, like a silicone mat or crafting mat, which won't leave impressions or stick to the resin.

The same caution applies when you move a long top for installation. It can be done, and contractors install epoxy surfaces away from where they were poured all the time, but be careful and keep any flexing to a minimum.