Unsaturated polyesters form this group. They are linear polymers (or drying oil alkyds) dissolved in the monomer styrene. The film is formed by co-polymerisation of the polymer and the monomer by use of peroxide, catalysed by metal ions in the form of Cobalt driers.

Due to the sytrene acting as a solvent, and also taking part in the reaction, polyesters produce high solids coatings that are hard, mar-resistant and glossy. As such, they are extensively used, both pigmented and clear, for wood finishing.

Because of their curing mechanism (called free radical) they respond to ultra-violet irradiation curing. As this produces no heat, it is ideal for wood and chipboard.

Where polyesters are used as body fillers, the pigmented resin contains the cobalt catalyst, and addition of the peroxide in paste form then commences the curing process.

The high solid of polyesters makes them useful, when reacted with amino-resins in a stoving process, as high build undercoats for OE systems.