can't give any info on fibreglass, however after rebuilding the rust in my car with metal plates and bogged over the top, in around the rear window, probably the thickest patch of bog in the car the paint has cracked.
i am not sure if this is from stress across the chassis or because bog does this when applied thick. as ive heard it can crack when applied in thick layers, but not sure the reliability of the source. just thought id mention it as something to watch out for, both the thickness and where you are putting this bog and the stresses it will come under