Styrene is both the solvent and crosslinking agent in polyester resins - the resins will all contain a specific amount of styrene in the formulation to give the particular viscosity needed for the end use of the resin.
It is possible to add more if you wanted to thin the resin down for any particular reason - generally you can add up to around 3 - 7% before you start to effect the properties of the resin. For instance you can convert a brush gelcoat to one thin enough to spray by adding styrene.
It is a very good solvent for oils and greases etc so can be used to degrease GRP or metals etc before you bond to it.
