Choose a release by what the mold is made of and what you’re pouring into it. Smooth-On says its silicone rubber molds usually need no release unless you cast silicone into them, but its Smooth-Cast 300 resin needs one on most other surfaces. For epoxy castings, TotalBoat names a mold release paste wax or an aerosol release.
Release agents, in each maker’s words
| Product | Releases | Mold or model surfaces | Notes |
|---|---|---|---|
| Universal Mold Release | Mold rubbers (polyurethane, polysulfide) from models; epoxies and Smooth-On liquid plastics from cured rubber molds | Properly prepared models; cured rubber molds | Not for silicone rubber, which Smooth-On says to seal with SuperSeal only |
| Ease Release 200 | Polyurethane elastomers, epoxy, polyester, RTV silicones, rubber, thermoplastics | Aluminum, chrome, RTV silicone, epoxy, rubber and steel molds | Aerosol; a light coat before each molding; marked for industrial use only |
| Ease Release 205 | Urethane rubber from cured urethane; silicone rubber from itself; epoxy and urethane resins; thermoplastics | Steel, aluminum, chrome, epoxy, urethane rubber and plastics, sealed plaster, sealed concrete, wax | Liquid; don’t over apply; dry 10 minutes |
| Ease Release 2831 | Polyurethane foams and elastomers | Aluminum, steel and elastomeric molds | Wax based, no silicone |
| Petroleum Jelly | Silicone from silicone, as in two-part molds | Porous surfaces | For Smooth-On silicone rubbers; thin 1 part to 4 parts mineral spirits by weight |
| One Step | Cured urethane mold rubber | Plaster, concrete, stone, brick, marble, limestone, wood, metal | Sealer and release in one; not on modeling clays |
| AquaCon | Concrete castings | Smooth-On rubber molds, latex molds, melamine, metal, sealed wood | Water based; more on concrete |
From each product’s page or safety data sheet.
When a silicone mold needs no release
Smooth-On says its silicone rubber molds usually don’t need a release agent for Smooth-Cast 300, and that using one will prolong the mold’s life. Silicone poured against silicone is the exception: for Mold Star 15 SLOW, Smooth-On calls Ease Release 200 a proven release for silicone from silicone or other surfaces. ArtResin pours its epoxy into silicone molds and says the cured resin peels out.
Seal porous models first
Smooth-On says porous models such as wood, plaster and concrete, and modeling clays that contain water or sulfur, may need sealing before Universal Mold Release. Its SuperSeal goes on in at least 2 coats, with the last left to dry for at least 1 hour, and Sonite Wax seals porous surfaces but may still need a release on top. Mold Star 15 SLOW won’t cure against surfaces that contain sulfur even when sealed, and latex, certain woods and fresh polyester, epoxy or urethane may keep it from curing; Smooth-On says a clear acrylic lacquer on the model usually prevents that.
Epoxy and fiberglass castings
TotalBoat says a mold release paste wax or an aerosol release for epoxies preserves the mold and extends its life when you cast small parts in its TableTop Epoxy, and names the same two for its High Performance Epoxy. An Epoxyworks article by a WEST SYSTEM technical advisor lists carnauba car wax, PVA (a green liquid from hobby shops), Aqua Net Regular Hold hair spray, which it calls nearly all PVA, and plastic packing tape. It says multiple thin coats of spray work better than a couple of heavy ones, and that wax and a spray used together help with the first part pulled from a mold.
The same article says to avoid mold releases that contain silicone with epoxy, polyester and vinyl ester, because they can contaminate the part, and that silicone sprays can contaminate a whole shop.
Before you paint or glue the casting
Take the release off first. Smooth-On says to remove Universal Mold Release from a casting with isopropyl alcohol or acetone, then prime with auto body primer before an acrylic paint. Its Ease Release 200 page says molded parts need thorough cleaning before staining, painting or bonding, and the Smooth-Cast 300 page says any release must be removed before bonding.
Smooth-On’s safety data sheet for Universal Mold Release aerosol says it is a flammable aerosol in a pressurized container that may burst if heated: keep it away from heat, sparks and open flames. Smooth-On’s Ease Release 200 page says to use it with room-size ventilation and to wear safety glasses, long sleeves and rubber gloves.