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A while back I read a surprisingly interesting book about materials science ('The New Science of Strong Materials, or Why You Don't Fall Through the Floor,' by Prof. J. E. Gordon of the University of Reading, UK) which devoted an entire long chapter to crack propagation.
According to Gordon, fiberglass has good crack-stopping properties -- basically, what will happen with one of your stress cracks is that it will continue to grow until it encounters a fiberglass strand running at right angles to it, at which time the crack will have to spread along the width of the strand. This spreads its surface energy over a larger area so that the crack stops growing. If meanwhile you've eliminated the source of the stress, the crack should pretty much stay put and you can postpone fixing it.
However, if you continue to apply stress to the same area, more cracks will probably go through the same process, which eventually leads to the "starred" or "checked" appearance you often see in impact-damaged fiberglass. Still, since fixing the damage will involve grinding out and re-matting a pretty large area regardless, I wouldn't think it would matter much whether you've got one crack or several, as long as they're restricted to the same area. It'll cost about the same to fix either way.
If cracks start forming in other areas, though, that'll drive up the cost of your bodywork and you might want to start thinking about taking the car off the road until you can have it done, to avoid having it get a lot worse and more expensive.
You might also have to ask yourself WHY you're getting so much stress transmitted to the bodywork. If the chassis has gotten all rusty and floppy, the fiberglass is just going to keep on cracking until you resolve the underlying (literally) problem.
posted by 24.3.2...
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