Vinyl esters are the “epoxies” of the polyester range and well worth the extra cost. The handling and performance characteristics of vinyl esters are similar to polyesters and it has been shown that a thin layer with a vinyl ester resin can provide an excellent barrier to resist blistering in marine laminates. If you are building on a male mould use vinyl ester in the final layer below the water line using a fiberglass tissue to assist with the build up. In a female mould it has to be the first layer backing up the gelcoat.