Here is some technical data that you may find interesting regarding the differences between Epoxy Resin and Vinylester or Poyester Resins. 

In the marine industry, liquid plastics, namely epoxies, polyesters, and vinylesters are used to saturate (wet out) the fibers of wood, glass, kevlar amarid, or carbon to form a fiber reinforced plastic (FRP).  To create a quality part, adhesion to the fibers is the most important factor.  Not all resins keep their grip on fibers equally.

Epoxy resin is known in the marine industry for its incredible toughness and bonding strength.  Quality epoxy resins stick to other materials with 2,000-p.s.i. vs. only 500-p.s.i. for vinylester resins and less for polyesters.  In areas that must be able to flex and strain WITH the fibers  without micro-fracturing, epoxy resins offer much greater capability. Cured epoxy tends to be very resistant to moisture absorption.  Epoxy resin will bond dissimilar or already-cured materials which makes repair work that is  very reliable and strong.  Epoxy actually bonds to all sorts of fibers very well and also offers excellent results in repair-ability when it is used to bond two different materials together.

Vinylester resins are stronger than polyester resins and cheaper than epoxy resins. Vinylester resins utilize a polyester resin type of cross-linking molecules in the bonding process.  Vinylester is a hybrid form of polyester resin which has been toughened with epoxy molecules within the main moleculer structure.  Vinyester resins offer better resistance to moisture absorption than polyester resins but it’s downside is in the use of liquid styrene to thin it out (not god to breath that stuff) and its sensitivity to atmospheric moisture and temperature.  Sometimes it won’t cure if the atmospheric conditions are not right.  It also has difficulty in bonding dissimilar and already-cured materials.  It is not unusual for repair patches on vinylester resin canoes to delaminate or peel off.   As vinylester resin ages, it becomes a different resin so new vinylester resin sometimes resists bonding to your older canoe. It is also known that vinylester resins bond very well to fiberglass, but offer a poor bond to kevlar and carbon fibers.  Do to the touchy nature of vinylester resin, careful surface preparation is necessary if reasonable adhesion is desired for any repair work. 

Polyester resin is the cheapest resin available in the marine industry and offers the poorest adhesion, has the highest water absorption, highest shrinkage, and high VOC’s.  Polyester resin is only compatible with fiberglass fibers and is best suited to building things that are not weight sensitive.  It is also not tough and fractures easily. Polyesters tend to end up with micro-cracks and are tough to re-bond and suffer from osmotic blistering when untreated by an epoxy resin barrier to water.  This is really cheap stuff.

Summary – Epoxy resin has far more to offer in its ability to flex, prevent delamination, and ease of use for repairwork. Using epoxy resin leads to better quality products.