Epoxy or Polyester--How do you distinguish?

[Bob Ensor]

Fiberglass Experts:

I have a small one-design sailboat built by a builder who produced some of these class boats in epoxy and some in polyester. (The builder is no longer building these boats.) How do I determine which resin was used to build my boat?

Thanks for your help,
Bob Ensor

[Tim]

If the builder is still around, even if they're not building your particular model, you should be able to get this information directly from them if you can provide them your hull number.

From a practical standpoint, it is difficult at a glance to determine if polyester or epoxy resin was used. Both tend to be the same greenish color when cured. Unsanded epoxy might be a tad more amber/golden in hue than polyester, but this is not a clear cut distinction. Some boatbuilders use a red MEKP catalyst with their polyester resins so that is is obvious that the resin is catalized and mixed properly; ordinary MEKP is clear, and leaves that greenish tone to the cured product. Therefore, if the glass looks redish, it is likely polyester.

If you do any grinding it's usually obvious which resin you have. Polyester, when ground, emits a very distinct styrene odor; epoxy does not.

Hope this helps. There's no definitive way to tell just from looking, other than a best guess using some of these clues.