If it has fiberglass in it, I prefer not to grind it, might as well toss a bag of asbestos around the shop, you will be breathing it for the rest of your time in the shop. I will do it outdoors, but even that doesn't thrill me. A really sharp hand plane will make short work of it. I use a bi-metal metal cutting bandsaw to rough it out, and I do that outdoors. One needs to work from fat to thin to avoid blowing out glass.