It is interesting to see and understand how molded fiberglass shells are made. So below is a video tour of the process.
when a molded fiberglass trailer is made inside the mold first thing that goes onto the sides of the mold is a release agent. That is not part of the finished surface. The next thing done to the mold is to coat it with the gel coat material. That is sprayed on in a thin layer. After that the "chop" which is resin mixed with chopped up strands of fiberglass threads is sprayed into the mold. Then it is left in the mold to cure before taking the parts out of the mold. So the gel coat amd the fiberglass structure are bonded together layers.
Go to 12:00 minutes on this video to see Gel coat being applied inside of a boat mold. Same process is used for the molded fiberglass trailers. Note the very bright red and green colors on the inside of the molds, that is the release agent that was applied to the mold so that the shell can easily come out of the mold after curing.
This video shows the "chop" being applied over the gel coat layer. It gets sprayed on with a special gun that cuts and mixes the fiberglass strands into the resin. Then another worker follows behind with a roller to compress and even out the sprayed on layer.