What should I clean sanded epoxy with before the next coat?

Use denatured alcohol or acetone, nothing else. After scuffing the cured epoxy with 220 or 320 grit, wipe the surface with a clean rag and denatured alcohol or acetone. Use a rag that won't leave lint behind, and keep wiping until every trace of sanding dust is gone. The surface must be completely clean and dry before you pour the next coat.

Do not use paint thinner (mineral spirits). If you've only gone over the surface with a damp rag, finish the job with denatured alcohol or acetone.

If you sanded out air bubbles or other defects, check that no white sanding dust is left inside the pits. If you can't wipe it out, sand a little further to open the bubbles up so they can be cleaned completely.

Once the surface is clean and dry, pour the next coat of UltraClear Bar & Table Top Epoxy directly over it. The new coat fills in the sanding scratches and cures clear and glass-like again.