Can I sand and polish Deep Pour Epoxy or a river table?

You can, but polishing isn't the finish we recommend. UltraClear Deep Pour Epoxy is softer than UltraClear Bar & Table Top Epoxy, so for a clear, glossy, durable surface, top coat the sanded Deep Pour with Bar & Table Top Epoxy. The new coat fills the sanding scratches and cures glossy on its own.

For a river table: pour the river with Deep Pour Epoxy to join the slabs. Once cured, remove the form, sand or plane as needed, and clean. Seal coat the wood with Bar & Table Top Epoxy, then flood coat the entire top and let the epoxy flow over the sides. It forms a seamless edge by itself, so there's no edge finishing or meniscus removal. After 3 days of curing, sand the drips off the underside. Full guide: https://ultraclear.helpscoutdocs.com/article/859-table-top-epoxy-deep-pour-epoxy-river-table

If you want the wood left bare, the other finish we recommend is Osmo Polyx-Oil (https://www.bestbartopepoxy.com/collections/tools-supplies/products/osmo-polyx-oil), commonly used on smaller river pours and surfaces that don't see much use. Bar & Table Top Epoxy gives the most protection and scratch resistance. If you buff epoxy next to bare wood, tape the wood off; we haven't had reports of buffing compound harming wood.

For a piece cast in a mold, either polish it (a very skilled person can use 3M Finesse-It 2, at your own risk) or, after demolding, scuff it with 220–320 grit, wipe clean with denatured alcohol or acetone, and re-coat. The next pour fills the scratches and looks like clear glass again.