: Just to clarify, laminating polyester resin IS supposed to cure
: hard, but it will not cure in the presence of air. It is meant
: to be used in molds that seal it from air exposure. Adding wax,
: or buying resin that contains wax ("finishing" resin),
: creates an air barrier on the surface that seals out air so it
: will cure hard. You can do the same thing with laminating resin
: by covering it with Mylar or polyethylene sheet.

Only the surface of laminating resin remains un-hardened. The purpose of this characteristic is to allow chemical bonding between the layers of a fiber reinforced laminate, and to do this more or less irrespective of how soon subsequent layers are added. The final layer in the laminate is applied with waxed resin, so the finished product is chemically bonded throughout, and the finished surface is hard. If non-laminating resin is used in initial layup, the quick hardening and waxed surface can defeat chemical bonding, particularly on large projects where completion of each layer takes a long time.

As Bill points out, polyester resin is a relatively poor adhesive, so the strength of a polyester resin laminate depends critically on achieving tight chemical bonding between layers.

AE