Bare-Metal How to Use Poly 74 and 75 Polyurethane Rubber Mold Compound


Porous surfaces such as wood, terra cotta, or plaster must be sealed to prevent rubber from penetrating the pores. Several coats of paste wax, allowed to dry and polished or melted paraffin, petroleum jelly, lacquer, shellac, paint, PVA (polyvinyl alcohol solution), and potters soap on plaster all work well for certain surfaces and mold rubber or resin combinations. Polyurethane rubbers bond tenaciously to shellac however, so if shellac is used as the sealer, release agent must be very carefully applied over it.

Bare plaster is best sealed with potters soap (Murphy's Oil Soap also works well) lathered into the damp surface of the plaster with a soft brush, rinsed and repeated several times, then polished with a soft brush or cloth. Soap is only to be used on plaster, it is not suitable for sealing any other material. If the plaster is dry, it should be wet for several seconds under running water prior to soaping.

Moist water clay is another special case. It can be sealed and released by simply spraying on a 10% solution of petroleum jelly in mineral spirits, made by warming and melting the petroleum jelly and stirring in the mineral spirits, then applied using a plant spray type bottle, and allowing the mineral spirits to dry. This prepares the water clay for pouring of most mold rubbers. If the water clay is completely dry, it should be sealed and coated with release as for terra cotta.

Release agent like Pol-Ease 2300 must be applied to nearly every surface before applying or pouring liquid rubber. It should be sprayed evenly and then gently brushed out with a dry brush to pick up any excess and to spread the release over any spots missed by the spray. The brush should be wiped with a paper towel periodically to prevent too heavy a coating being left on the surface. Too much release can cause pinhole defects in the surface of the liquid material poured against it. The coating of release should be allowed to dry for about fifteen minutes prior to pouring. Silicone-based release agents like Pol-Ease 2300 may interfere with the cure of silicone mold materials. Petroleum based products often work well as releases for silicones.

In every case where there is any question about the compatibility between the rubber and the prepared model surface, a test cure should be made on an identical surface to determine that complete curing and good release is obtained. Some materials such as sulfur-containing plasticenes and wood knots inhibit curing of some rubbers.

Porous models must be vented from beneath to prevent trapped air from forming bubbles in the rubber. A hole at least 1/4 inch diameter should be drilled through the base board into the porous model. Of course, the base of the model must be sealed around the perimeter with plasticene or glue to prevent liquid from leaking out under the model and through the vent hole.

Sideboards or shells should be securely fastened and sealed to the base to prevent leakage and all sideboards and shell interiors should be well sealed and released. Petroleum jelly is excellent for most non-critical surfaces like these, unless the material gets warm enough during cure to melt the petroleum jelly. Pol-Ease 2300 is more heat resistant, but is not self-sealing, so surfaces beneath must be well sealed.


Have all materials at the same temperature. Warm room temperature, 70-85°F, is best. Remember, it may take 24 hours for a five gallon pail of material to warm up to room temperature in the winter.

Have all materials and equipment clean and ready. You do not want to have to go searching for something after you have already mixed the material because time is critical to obtaining good results.

Stir individual components before use if recommended. Some materials separate and will not cure properly if separated material is used.

Weigh and measure accurately. Weighing on an accurate scale is best. Only materials with a mix ratio of one to one by volume can be measured by volume. We do not recommend trying to measure any other mix ratio by volume.

Close containers tightly after use. While most materials are usable at least six months to a year after shipment if unopened, some components can absorb atmospheric moisture, thus should be used up as soon as possible after the container is opened. Leaving caps off will cause deterioration even faster, sometimes within a few days.

Time your actions with a clock. Start timing when you start mixing. Try to have material poured and in place before half of the working time has elapsed so there is plenty of time for bubbles to rise away from the mold surface.

Mix well, but avoid whipping air into the mix. Scrape the sides and bottom of the mixing container thoroughly several times while mixing. A Poly Paddle is invaluable for good mixing.


Using rubber molds with attention to the following points will prolong mold life and produce better castings.

More rubber molds fail due to handling mistakes than by wearing out. Proper design and careful handling and attention to detail will prolong mold life.

Plaster castings will be free of bubbles if the rubber mold is wet with Pol-Ease Mold Rinse or with a 1-% detergent (e.g. Ivory Liquid) solution in water when the plaster is poured. Wet the mold by dipping or spraying, do not allow to dry. Pour the plaster on the wet surface. Please note that detergent and soap are chemically different. Do not wet the mold with soap, Ivory Liquid is a detergent.

Pol-Ease 2300 is an excellent release for Polyurethane and Epoxy resins with Polyurethane molds. Pol-Ease 2300 should be applied as a light even spray then carefully brushed with dry brush to pick up any excess and cover any missed spots.

For long mold life, avoid exposing molds to strong solvents, oils or cleaners. Such products affect all rubbers. Sprayed release agents reduce solvent exposure as compared to brush application. Store molds in their original shape undistorted, out of direct sunlight, in a cool, dry area. Molds can be stored in their mold shells, but porous shells such as plaster should be sealed with shellac to prevent absorption of oils from the mold by the plaster.

Do not allow molds of one type of rubber to remain in contact with those of other rubbers as migration of oils or plasticizers from one to another can cause swelling, shrinkage, or distortion.


Paper towels for wiping up are a must. Tools should be wiped clean before plastic or rubber is hard. Denatured ethyl alcohol, acetone, or MEK are good cleaning solvents, but are highly flammable. Work surfaces can be waxed or coated with Pol-Ease so hardened material can be removed.


Cab-O-Sil can be added to any of these products to thicken them to a brush consistency. Add Cab-O-Sil until desired thickness is obtained.


Read and understand the Material Safety Data Sheet (MSDS) label, and Technical Bulletin. All Polytek products can be used safely if simple precautions are taken as recommended in these sources of information which are provided with every product. All products should be used with good ventilation. Skin and eye contact, ingestion and breathing of dusts and vapors should be avoided.

Use necessary or recommended equipment such as gloves, dust masks, eye protection, closed shoes, adequate clothing and (rarely) respirators equipped with cartridges for organic vapors.

Follow directions. All too often, projects are spoiled because in enthusiasm for completing a project, important directions were not followed. Please read all instructions thoroughly and call if you have any questions, before starting your project.

Uncured Polytek compounds may cause skin or respiratory irritation or sensitization if improperly handled. Avoid skin and eye contact with uncured material. If skin contact occurs, remove with waterless hand cleaner or alcohol, then soap and water. In case of eye contamination, flood with plenty of water and call physician. Use only with adequate ventilation such as a large open room with air movement. If there is any doubt about adequacy of ventilation, a respirator with cartridges for organic vapors should be used. Polytek products are not to be used where food or prolonged body contact may occur.