The ideal people to ask would be Smooth-On.
There is not much information in your question, so I will ask: Do you intend to use the same 326 urethane resin with the fiberglass? Do you intend to use another type of resin: Polyester. Epoxy?
Urethane, once cured, does not accept additional layers well. (that is layers of Urethane, Polyester, or epoxy) They all tend to de-laminate.
As a general rule: Don't mix materials. If you are going to use urethane as a surface coat, back it up with more urethane resin. The second layer will adhere better if you give the first layer a sanding. It will also ruin a brush every time you use on, because you can't clean cured resin out of a brush. (from the spec-sheet, 326 has a pot-life of 9 minutes)
If you are going to use polyester resin for the fiberglass, also use polyester gelcoat.
Epoxy resin (recommended) doesn't need a gelcoat, but is more difficult to laminate than polyester.
Lastly, Vinyl-ester resin is a nice half-way point between polyester, and epoxy.