Use an angle grinder with an abrasive sanding disk and grind back all of the damage area completely, Wipe clean with Acetone then use a Marine grade filler with fillers or thickeners if needed blend into a manageable consistency for horizontal or vertical surfaces, consider an above or below the water line product as the case may be, most of this Marine Grade filler is found in the local Marine stores.
However if you are going to use Gel Coat over this patch you must use a polyester base filler or resin not Epoxy (Gel Coat will not adhere to it). For paint I don't thing it matters ?
Check http:\\
www.youtube.com there are some excellent videos on patching and flaring or working with glass in general.
Simply fill it let dry and sand to perfection and you are done, blame the itching on the summer heat :)
Not very hard, I never done stuff like this and was able to restore my V.
Here are some serious filling, sanding and Gel Coating before and after done in my driveway during my restoration and transom repair. Good luck and keep us posted.
