RE: Advice on producing Fiberglass fuse from mold.
Some other info about polyester vs. epoxy...
One of the big issues is the shrinkage of the resin. Resin shrinkage causes fabric print-thru (surface blemish), warping of molds and parts, and internal stress within the laminate the can lead to micro-fracturing and reduction in toughness. The shrinkage of epoxy is significantly lower than polyester.
Most of my parts pop out of the mold painted and ready to go. I spend a lot of time creating molds with stellar finishes so the painted parts come out of the mold with stellar surfaces. This is a big time and money savings. If the resin shrinks too much the fabric will print through to the surface of the part or mold and ruin that finish. This is a very common occurrence with polyester resins (and epoxies if you pop the part out of the mold too soon or cure it too quickly with heat). This really stinks if you've made a mold and end up with the fiber backing telegraphing through to the mold surface. Now you have to polish the thing if you want to remove it. If you're painting your parts after molding then may not be such a big deal since you can sand the blemish out of the surface.
Who likes molds that warp and don't line up properly? This is a pretty common occurrence with molds made from polyester due to it's higher degree of shrinkage. Again, if you're painting after the part is produced then these irregularities can be repaired. If you want to get pristine parts out of the mold then the mold must hold it's shape. Using a low shrinkage resin like epoxy can make a significant difference stability of the mold (if the mold is constructed properly).
Thin flying surface parts made with polyester are also much more prone to warping over time. A razor thin racing foil with a warp is now worse than the Clark Y. If the part is thin, epoxy will make a more stable part.
The shrinkage of the resin will also create internal stress which can promote micro-fracturing when the composite is flexed. Ever seen a gel coat, which is polyester based, all full of cracks after an impact....or just from sitting around? Laminates made with epoxy are simply tougher. They can be stressed much further before these micro internal cracks form. For this reason epoxy laminates will fatigue much more favorably than polyester resins and have a higher impact strength.
I also like my internal organs. The off-gassing of curing polyester isn't good for you. Most manufactures recommend the use of a respirator while using polyester, while most epoxy manufacturers state that a respirator isn't necessary if the ventilation is adequate. Almost all modern epoxies are also VOC free. Skin contact with either resin isn't good and using solvents to clean it from your skin will carry it right into your blood stream. Bad news.
There are also other reason like increased HDT, Tg, exothermic reactions during mold construction (generated heat), shelf life, bonding strength, extended cure times, and inter-laminar strength that make epoxy a favorable choice for me.
Ohhhh...and my wife hates the smell of polyester.
That all being said, polester resin works, is cheap, and arrives at physical properties much sooner in the cure than epoxy. If you can't afford epoxy then using polyester to make a part or mold is better than not having a mold or part at all.