Glazing Rubber Products Plastic Horseshoe Shims are made to simplify leveling problems wherever they may occur. Applications for GRP Horseshoe Shims include installation of windows, doors, curtainwalls, storefronts and insulated glass units. GRP Horseshoe Shims are molded from a fire retardant engineered co-polymer plastic. They are classed as slow burning with no toxic fumes. They have a compressive strength of 10 to 12,000 psi with no fracture even at 35,000 psi.