Which epoxy should I use to fill cracks, knot holes, and voids in wood?

It depends on how deep the void is.

Less than 1/4" deep: use UltraClear Bar & Table Top Epoxy. It self-levels to 1/8" and can only be poured 1/8" thick per layer, so build deeper spots up in 1/8" layers spaced 4–6 hours apart.

1/4" deep or more: use UltraClear Deep Pour Epoxy. It can be poured 1" deep in a single pour. For deeper voids (a 2" crack, for example), pour it in 1" layers every 12–24 hours, up to 6" total.

For a bar, table or countertop you will normally need both products. Bar & Table Top Epoxy seal coats the bare wood first, Deep Pour fills the deep voids level with the surrounding surface, and Bar & Table Top goes on last as the 1/8" flood coat that gives you the hard, durable finish. Deep Pour can't be used for anything under 1/4" thick, so it can't be your seal coat or top coat.

Either product can be tinted with Pigmently pigments when filling voids. Wood filler also works to level voids before epoxy, but matching it to the surface is up to you.

Deep Pour's smallest size is the 1.5 gallon kit. If you only have a cup or two of voids to fill, you can reduce their depth with wood filler or caulk (test first) and fill the rest with Bar & Table Top in 1/8" layers.

Guide to using both products together: https://ultraclear.helpscoutdocs.com/article/859-table-top-epoxy-deep-pour-epoxy-river-table