Bob,

The Gelcoat can be thinned with acetone - you can even thin it enough to spray it if you wanted. I believe the key to getting a smooth finish on it is to coat it with something after application. This keeps the air from affecting the cure. For small repairs, use a piece of saran wrap stretched tight. If the bite out of the trailing edge of your board is more than .1875" or so, you will probably need to put in some glass. Some Gelcoats require to be coated so they can cure. Some other folks spray a PVA mold release over the gel to facilitate a cure.

For a sizeable 'bite' I would first take a dremel and clean up the edges and then taper them so I have a spot to build up some glass on. I then take a piece of duct tape with wax paper on it (leaving adhesive of the duct tape all the way around the wax paper) and place it over the area on one side. Depending on what the area looks like, I might first lay up some glass and epoxy on a piece of waxed lexan to get a solid sheet that I can cut some pieces from. Then trim and epoxy a few pieces of the glass in the wound. If the wound is thick, I would only use this on the outside edges and would build up the inside with un-epoxied glass and more epoxy. For the final finish, I mix the epoxy with some micro-balloons to achieve an easier sandable finish.