Most of them use polyester resin. Polyester has a distinct smell and you really notice it when you drill or cut the part. The only strength to the whole structure is provided by the resin and fiberglass fabrication. One is useless without the other. The parts are built from a "plug" with one or two layers of glass/resin layup.
The primer is only there to cover imperfections and provide a smooth surface for the paint. It has no structural properties at all. Too much primer and the paint job will crack link crazy.