A gelcoat and fiberglass-based mold is an excellent option for smaller production runs, typically up to 100 parts. While not as durable as aluminum or steel, they are adequate for many types of parts and can be created with much simpler tools and cheaper materials.
Begin with a finished and polished plug. We prefer to use TR wax, but you can choose to spray a layer of polyvinyl alcohol (PVA) instead. These release agents ensure that the mold won't stick to the plug during the de-molding process. Use compressed air and a tack cloth to remove any dust from the surface before spraying.
Prepare a spray gun. We recommend having a dedicated gel coating spray gun since it’s a messy process and you don’t want to contaminate a gun used for paint or clear coat. Don’t spray more than 500ml of material at a time since it is likely to start curing, or 'kicking', before you can spray any more than that. If temperatures are particularly hot you may need to reduce the amount of catalyst you are using and spray in even smaller batches.
Gel coat is thick, so you want to spray it as quickly as possible. Adjust the gun to keep the air pressure high and unscrew the fluid adjustment screw as much as possible. Begin with a mist coat and then lay down successive coats of material, evenly coating all the way to the edge of the plug.
After 15-30 minutes the gel coat will start to kick. As soon as you notice the material thickening as it sprays from the gun, immediately stop and pour out any remaining gelcoat. Disassemble the gun and clean it with brushes and acetone. It is critical to thoroughly clean the gun between each batch of material as any leftover residue will make the gun spray poorly and eventually ruin it.
Take a mil gauge reading [check back soon for our post on how to read one!] from the flange. 15-30 mils is enough. The gelcoat will not fully cure while exposed to air, but it does need to partially cure before applying the fiberglass backing. This usually takes about 45 minutes – it is ready when you can gently touch it without getting any material on your finger but it still soft enough to leave a fingerprint.