grind and bevel the edges of the crack with 36 grit,roughen up the area around the crack about 2 inches with 80 grit on a D/A,mix fiberglass or epoxy resin whichever one you can get,apply two layers of fiberglass matt,brush enough resin on the matt jst to get it wet, not running and sagging all over the place,brush off any excessive amounts of resin,the closer and tighter the fiberglass matt to the repair surface and the less resin, the stronger the repair, let it sit for few hours and cure,grind and sand smooth with 80 grit,usually if it's a stress crack,you may want to do the same on the underside if the access is possible,after sanding,skim the area with thin coat of filler or glazing putty, block, prep and prime, oh and don't forget to wash the panel, dry it off then wipe it with wax and grease remover before doint any repairs, God forbid I forget to tell you that, "wax and grease remover police" will be on my case