Can I add more epoxy later if I ran out during a flood coat?

Yes. How well it blends depends on when you ran out.

If you ran out in the middle of pouring a flood coat, expect a seam where the old epoxy meets the new. Each batch of a flood coat needs to be poured within 20 minutes of the previous batch so they flow together into a seamless finish. Bare spots left after a flood coat mean the same thing: not enough material. In both cases, prep the surface as below and re-pour a complete flood coat with enough epoxy for the whole area. One gallon of UltraClear Bar & Table Top Epoxy covers about 16 sq ft as a flood coat or 48 sq ft as a seal coat; the coverage calculator gives the exact amount for your dimensions.

If the coat is complete and you just need another layer, no problem. Within 4-10 hours of the previous coat, pour the next layer with no prep; the layers bond as one. Past 10 hours, lightly scuff the surface with 220 or 320 grit sandpaper, then wipe it with a clean, lint-free rag and denatured alcohol or acetone until all dust is gone. Do not use paint thinner or mineral spirits. The next pour fills the sanding scratches and cures crystal clear again.

If you ran short during the seal coat, scuff and clean the same way, brush on another seal coat until no bare wood is exposed, wait 4-6 hours, then pour the flood coat.

Coverage calculator: https://www.bestbartopepoxy.com/pages/epoxy-calculator