Though SMC does not have visible fibers on its backside, it does have reinforcing fibers on its inside. When damage extends through the material, some fibers will be
damaged and they, as with damaged FRP, must be removed. This can be done by using a die grinder or a small precipitating saw. The damaged SMC must be removed before the repair is completed. Most manufacturers of SMC repair material recommend the use of a scrap piece of SMC for the backing, though there are also backing reinforcement tapes and other materials that can be used for the backing repair.
Though the recommendations of the adhesive material manufacturer should be followed, a general repair procedure to follow is listed below:
1. Wash with soap and water.
2. Clean with a wax and grease remover.
3. Remove paint with a grinder and #80grit sandpaper. The paint should be removed to approximately 3 inches beyond the repair area on both sides.
4. Bevel the edges of the repair area to a 30° bevel on both sides.
5. Remove sanding dust.
6. Prepare a SMC backing from a scrap piece of SMC. It should have the same contour as the repair area. The repair piece should be washed, cleaned, and have the paint removed with #80 grit paper.
7. Prepare the recommended product used to bond the backer. This may or may not be a different product than the one used to repair the front. Some SMC adhesives require the use of heat to catalyze the adhesive. The manufacturer's recommendation for time or heat should be followed. Allow to cure.
8. After the backing has been applied and cured, clean the front and apply the repair material to the front. Some manufacturers may recommend the use of a “pyramid” front repair, which means that repair material and reinforcement tape are stacked upon each other until the level of the repair reaches the surface of the defect.
9. Sand the repaired area to contour. Then apply primer filler, and block.
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