What types of finishes can be applied to a mold in advance of a substrate?

Like if I was pressing a clay-like cardboard slurry into a prepared mold preped similar to a gel coated fiberglass mold, what might work well to seal or add a surface finish?

I could make a vacuum form for a thermoplastic wrap, but coating a mold first has long been a curiosity of mine. Foils and leaf seem obvious, but what about coatings?