When fiberglass first became available for making bows, Fred Bear got right into it. His first prototype bows had glass only on the back of the bows. What he found was that the glass overpowers any wood and will cause the belly to fail. At the time, Fred had only woven glass to work with. He discovered woven glass failed on the belly because it crinkled up under compression as the fibers were not straight. This was the driver to develop unidirectional glass. Until he got the unidirectional glass thing working, he put aluminum on the belly.

Short answer is yes, if you want the bow to last, either put glass on the belly too, or don’t use glass at all. Of course, as with all things bows, I am sure there is a way to do it. Just need to figure it out. The real question is, how many bows are you willing to make to get one that works?

Years ago I built a couple of Osage board bows backed, back only, with fiberglass. I did not use a heat oven but simply clamped the lams to the limbs for gluing with epoxy. While I can’t recall the nature of their failures, I do recall that it didn’t work out at all. Sure would make bow-building easier if we could back wood with glass on one side and without a heat oven. My next bow will be an Osage selfbow, as the few of those I made were far superior to board bows no matter what I backed them with.

But hey, this isn’t rocket science and nobody’s life depends on the outcome, so why not experiment and have fun. So long as you have the time and materials to spare …