ORIGINAL: wyowindworks

An epoxy resin can still get hard with an improper mix ratio. It may not be at it's designed performance level though.....especially in respect to Tg and HDT. When mixing epoxy surface coats for molds an accurate mix can make a big difference in how the mold releases. If there are lots of open molecular sites on the mold surface, due to low cross-link density of the surface coat, then the fresh laminating resin used to make the parts will try and chemically link with the open sites on the mold surface. Wax is unable to block this chemical reactivity and PVA will be a must for a very long time.....sometimes for the life of the mold.

Hobby epoxy bonding resins often have a 1:1 mix ratio. Laminating resins do not....at least I haven't use one.

''Finishing Resin'' in the composites industry refers to a polyester resin that contains a wax. Polyester resin by itself has a air inhibited cure. This means that the outside surface of the resin that is exposed to the air will remain tacky. This is great if you are going to add more layers because it enables you to have chemical linking between the two applications. The tacky surface is a problem if it's the last layer and you need to sand and/or add paint. The solution is to add a special wax to the resin. The wax floats to the surface and seals the curing resin from the air. This allows the resin on the surface to cure nice and hard. The wax needs to be removed before one would do any painting. A polyester resin that comes with the waxed pre-mixed is called a finishing resin. Why Pacer calls their epoxy laminating resin a finishing resin makes no sense. In my opinion this has caused a lot of misunderstanding in the hobby world. Some epoxies will experience amine blush if the humidity is high. If you work in an environment with higher humidity or high carbon dioxide levels then you want to use a non-blushing epoxy resin. Blushing is not a problem in molding applications because the part surface is sealed from the air by the mold.