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Clear Polyesters Vs. Clear Urethanes
In short, crystal clear polyester resins are more economical, but more difficult to use and have less desirable cured properties. They are not UV stable and thus will yellow over time. Polyesters release a large amount of heat when they cure (exothermic reaction). If the casting gets too hot, it will begin to degrade and may bubble, turn brown or crack. This heat released on curing is the reason that we recommend only casting 1/4″ thick layers. You could probably get by casting 2-3 layers per day. There is a large amount a shrinkage associated with the cure of polyester and thus you may have to polish and buff the surface to obtain a shiny surface.
Crystal clear urtethanes are about 3 times as expensive for the same volume, but are UV stable and much easier to use. The mix ratio is 1:1 by volume. The exotherm is much more controlled and some products have been cast up to 12″ in a single pour. The shrinkage is negligible, and as long as a smooth, polished mold is used, there is no need for buffing the surface.