Has anyone successfully used fiberglass in a large enclosure instead of polyurethane?

Here is what we came up with, and it wasn't pretty... it raised up, cracked and was this ugly brown color (I said it was too much, but Chris begged to differ... well looky who was right )


Any tips? Or should we just go back to putting on a bunch of coats of polyurethane? I don't have a problem with that, but using fiber glass would be so much easier.