the factory spoiler is a metal core with a urethane outer. The urethane was poorly sealed and the metal poorly prepped so the spoiler "wicks" water inside and the metal rusts. The rust expands causing the urethane to crack and as you know, rust is near impossible to stop, especially when you can't get to it. Filling the crack is just a temporary repair. It will return and not too long down the road. I too was leary of the fiberglass spoiler for the same exact reason you are. My cousin's 89 Formula was rearended and he got the insurance company to buy one of the glass ones. I was extremely impressed with the quality. You can not tell a factory spoiler from the glass spoiler unless you touch it. you can be inches from it and never know it's unoriginal. It's also a third the cost of a new GM if you can find a supplier that has one on the shelf still. Don't waste your time with repairing it, you might find a good used one but even this is difficult. I had 2 good used ones I put on my TTA's and both ended up less than perfect once I sanded them down, painted them and put them on. I had one perfect one, spent a month sanding it, had a friend paint it went to bolt the 3rd brake light to it and the nuts for the bolts spun inside the spoiler causing 2 dimples sticking up right in the middle of the spoiler on top. I was sick. the second one was uncracked but the urethane spiderwebbed on top and it came through the paint. The spiderwebbing is usually thought to be the paint/primer but it's not, it goes right through to the urethane and I don't know how you'd fix it short of putting some sort of resin coating on it.
