If you are going to glass the joint as echobot suggested using 30 minute epoxy, once you get the cloth wrapped completely around the joint and you put the epoxy to it, lay a piece of waxed paper on top of the epoxy and with an old credit card, or something of this type, go over the joint working the epoxy in real good and feather outward from the joint. The waxed paper and credit card will give you a nice smooth finish that shouldn't need any sanding before you cover it.

Hope this helps,