Do I have to seal river edges before pouring Deep Pour Epoxy?

Yes, in nearly every case. Brush a seal coat of UltraClear Bar & Table Top Epoxy onto the river edges and any other bare wood the Deep Pour will touch, including worm holes and knot holes. The seal coat stops air from escaping the wood and rising into the pour as bubbles. Three coats of polyurethane on the sides is not a substitute; you still need the epoxy seal coat before the Deep Pour.

How many seal coats depends on the river. If you are coloring the Deep Pour, one seal coat is usually enough, because any bubbles that escape through it are hidden by the color. If you are pouring clear Deep Pour, apply multiple seal coats to fully stop trapped air from escaping.

The one exception is a river with enough pigment to be fully opaque. Some users skip the seal coat there because bubbles will not be visible. If your river is clear or translucent, seal the edges first. Test your color amount on a small scale before deciding; there is no set ratio of color to resin.

You do not need to use the epoxy to seal the underside of the slabs, though you can if you like. Most users seal the bottom with something like an oil-based polyurethane. If you do seal the bottom with epoxy, it does not need sanding, since you will not pour over it.

River table guide: https://ultraclear.helpscoutdocs.com/article/859-table-top-epoxy-deep-pour-epoxy-river-table