Applying gel coat finishes to fiberglass or composite assemblies is a combination of both procedure and technique. Any time you use an epoxy or resin-based material, be prepared to work in a clean, dry and temperature-controlled environment. Changes in humidity wreaks havoc on curing resins and dust in the air can ruin the finest wet finish. Always wear a breathing mask when applying gel coat. This not only protects you from the chemical fumes but also protects the gel coat finish from sneezes, coughs or anything else expelled when breathing or talking.
Make sure the surface is completely cured and free of grease and dirt. Before gel coating, clean the entire surface thoroughly with a good degreaser cleaner such as Formula 409 or Fantastic. After using the cleanser, buff the surface dry with a soft cloth. Then allow the surface to stand for a few minutes to allow the last bit of moisture to dry. Avoid fingerprints on the surface. Skin oils can cause gel goat resin to cure improperly and lead to unsightly blemishes. Always wear latex gloves when working around gel coat. Again, it not only protects your hands but also protects surfaces from contaminating skin oils and hair. Also, before doing any gel coating make sure all fans, ventilators and air draft sources are turned off or closed. Wafting bits of dust landing in wet gel coat can ruin the finish.
Gel coat always works best when sprayed on a surface. Applying an even, uninterrupted layer makes the surface continuous and blemish-free. Gel coat works best with a 2mm spray nozzle but larger nozzles can work if you want to cover more area with each pass of the spray gun. The only drawback is making sure the entire swath of gel coat is evenly thick. Wide swaths are more likely to be thick in the middle and thinner on the edge. A 2mm spray produces a nice even swath across the surface. Avoid multiple coats when spraying. One good even coat is better with gel coat than treating it like paint. Gel coat will be opaque (if desired) with one even spray coat. When finished spraying gel coat, make sure there is plenty of acetone to clean all components of the spray gun and holding tank. Gel coat is a resin that will harden and adhere to metal surfaces. Once it is cured, even a long soak in acetone may not clear the spray gun completely.
Applying gel coat with a brush is not recommended but can be done if necessary. Do not apply gel coat like paint. Apply gel coat with a full brush in long, even strokes. Try to have the brush travel from end to end of the surface, if possible. Try to avoid lifting the brush from the surface once you start. Again, the objective is to apply even thickness to the entire area. Brushes do not achieve this as easily or quickly as spray applicators. Be sure to have acetone ready to clean the brush and your gel coat container.
Gel Coat Hardening
Remember: once you add the hardener to gel coat you have a limited amount of time to get it applied. Consider mixing gel coat in the spray applicator container. This allows more time for applying the gel coat before hardening. Transferring the gel coat from one container to another takes precious time. It is important to finish applying your gel coat long before the manufacturer’s estimated drying time. If you are applying gel coat to a large area (a fiberglass auto body panel, for example), make small batches and apply. Do not mix up a large amount of gel coat that you cannot apply before hardening. That’s a great way to end up throwing away a lot of expensive material.
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