Making a traditional fiberglass part is accomplished by working from the outside surface into the part. A clean and waxed mold is placed in a clean room where contamination will not interfere. Then gelcoat is sprayed on as the first layer, and followed by the rest of the laminate. The gelcoat application process is much more art than science.

The objective of gelcoat is to create a uniform thickness across the part, and have it be 18 to 25 mils thick when wet. As it cures, this thickness is reduced when some of the chemicals evaporate off. Spraying gelcoat onto a mold returns the best quality finish, but it

may also be brushed in areas that are difficult to spray. Achieving a uniform thickness is difficult at intersecting corners, deep narrow areas, and difficult-to-reach sections of the mold.

The gelcoat is typically applied in three passes and allowed to “gas off” in between coats. This allows for some of the chemical evaporation to start, and can lead to problems if not done correctly. These passes are also usually bi-directional, where the first and third pass are in one direction and the second is in another, again to help achieve uniformity.

Areas of excessive gelcoat thickness can lead to cracking and surface finish problems. Areas of insufficient gelcoat thickness can “alligator” which is a surface flaw requiring extensive repair.

Gelcoat Application is a tricky part of the process and the manufacturer of the application equipment and the raw material is the best resource for best practices. Experience is the best formula for great results.
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