[jiminpa]

depending on exactly what you have, here are a couple ideas.
if you have actual cracks that are progressively getting worse, drill a hole just at the very end of the crack. This will help to stop the progression. You can buy fiberglass resin & fiberglass cloth at most auto parts shops, patch it from the inside with the cloth, then, after it cures, carefully fill the cracks from the outside with just the resin to reduce the mess. A small paint brush works well.
Show us some close up pictures of the problem to be fixed...
You might also want to try something called "Dura-Glass".
Regardless of what you do, it is best to drill the end of the crack to stop it from getting worse.