when i repair spider cracks in new corvettes i use this stuff, fibertec #633 http://www.evercoat.com/productDetail.aspx?pID=13 it has kevlar fibers in it that helps strengthen the repair area.
you should completely grind out the cracks in the glass then spread this stuff in the area you just ground out. then i use this stuff, metal glaxe #416 http://www.evercoat.com/productDetail.aspx?pID=1 on top of that. this is the only products that will not bulls eye on me when i repair corvettes.