Smooth-On URE-FIL® fillers disperse easily in Smooth-On materials. They can be added to materials in different proportions to achieve a variety of working properties and effects. Some fillers are designed for cost-savings, others are designed for specific casting effects or improved physical properties. Other URE-FIL® fillers are designed specifically for thickening materials for brush-on or trowelable application.
URE-FIL® 7 mixes well with urethane resins and castable epoxies and will improve dimensional stability (reduce the shrinkage rate of some resins, for example). It is also often added to materials to make them flame-resistant. URE-FIL® 7 can be mixed with Smooth-Cast® 325 and SO-Strong® color tints to create marble effects. The maximum amount that you can add to a resin before compromising the physical properties of the cured plastic would be 1A:1B to 2 parts filler. A mix ratio of 1A:1B to 3 parts filler will make the plastic trowelable, but the cured plastic will also be brittle.
URE-FIL® fillers must be thoroughly dispersed in liquid rubber or resin to a uniform consistency. When adding large concentrations of filler, it is recommended to add the filler to completely mixed rubber or resin - combine both parts A and B of rubber or resin, mix throughly, then add filler. Mix the filler in and confirm that there are no lumps of filler present. Often, employing the ‘double-mix and pour’ technique can produce better results.
Guarding Against Moisture - All powder fillers will absorb atmospheric moisture over time which will react with urethane materials. If you experience bubbling or foaming in your resin/filler mix, your filler probably contains moisture.
Remedy: Line a cookie sheet with foil and spread filler evenly over the cookie sheet at 3/8”-1/2” thickness (0.95 cm-1.25 cm). Place in an oven on warm setting (100°F / 38°C) and bake for two hours. Remove cookie sheet from oven (don’t burn yourself!) and redistribute filler on cookie sheet. Place back in oven and bake for two hours. Let filler cool to room temperature before mixing with urethane materials.
Storage - To prevent moisture absorption, use filler as quickly as possible after purchasing. Decant filler from container and replace lid immediately...do not leave container opened for more than a minute. If filler will be stored for prolonged periods, obtain dessicant pouches at your local hobby/arts and crafts store and place one or two dessicant pouches in container. Replace used dessicant pouches every other time you open container. Make sure container lid is secure before storing.