Resin choices for casting: polyurethane, epoxy and polyester compared

Polyurethane casting resin (Smooth-Cast 300 class): mixes 1:1, cures in minutes, opaque white, takes pigment well, cheap per part. Moisture sensitive; a humid day gives foamy castings. The workhorse for parts and figures.

Epoxy: slow (hours), crystal clear, great for embedding things and for art pieces, exotherms badly in thick pours (use a deep-pour grade). Yellows in UV unless stabilized.

Polyester: cheap, clear, stinks, shrinks, cracks in thick sections. Used for fiberglass; mostly avoid for casting.

Measure by weight on a scale unless the datasheet says volume. Mix slowly, scrape the sides and bottom, pour down one side of the mold.