Not trying to steal Smooth's thunder, I'm a fan of his work, and not sure where you're describing but most fiberglass failed repairs are from undone prep. You're fighting a stress-riser and it takes some thought from virtually an engineering standpoint and some additional prep to overcome it. In a nutshell "spread out the load". And prep, prep, prep.
On a flat panel, the parent material should be ground back in a wide, shallow vee, I mean WIDE... rule of thumb is 10X the thickness of the panel, more if it's a large piece. Where possible, for max strength you can go half-way thru on one side, grinding back and make your repair and then repeat on the other side.
Where you have to tie-in a radius that's too small for the cloth to easily conform to, as next to a moulding recess, one trick is to cut cloth diagonally in 1/8" hacks to create a heap of short fibers. [Carefull, you now have an army of miniature porcupines that love skin, and for God's sake don't breathe them.] Mix in your ready-to-go epoxy to the consistancy of peanut butter to fill the spot. [Be careful mixing so not to get air whipped in.] After setting, smooth & roughen and your next layer of cloth will have a nice wide bite. Or cut out the recess entirely, rig up a temporary backing and flat-panel it.
In *any* case, the surface has to be *roughened and clean* for the material to get a good bite.
Don't know how SmoothGT did his cool deal, hope he'll reveal some of his secrets. [Way to go Smooth, I always thought the beltline was too busy-looking.] Good luck.
