Re: Alternative to flap diskc for sanding
From my experience, the flappers do what they are supposed to do, the problem is perception. Initially, they work great because the only thing exposed to the work surface is virgin sand paper. Once that wears down, you are essentially wearing down to the next "line" of grit by using friction to wear away at the backing paper. The flappers can last forever, but they end up taking longer to use. I thought I was going through the flappers too fast on my boat, but when I put one back on the grinder and went at it again, it was still cutting, just at a slower rate.
I also have the fiber reinforced sanding discs and would agree that they are superior to the flap discs as far as pure cutting power goes. The flapper has a shallower learning curve and gets you into much less trouble, so I would consider them as grinder "training wheels" for anyone that hasn't worked with fiberglass before. It's a slower process, but you have more control over how much material you are taking off.
Once you've worn through a couple of flap discs, then graduate to the singles...and you'll know when they are worn out.
Don't cheap out on paper backed discs though, get the reinforced ones. You get what you pay for.
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