Make the patches 1 inch square and wet them out. Let them cure and then sand them down to fair. Casey aside, for 3/16", I would probably just use chop or strand cut down with nail scissors and put THAT into thickened epoxy, and then fair that, and then coat the entire rudder in two-part barrier coat, and then paint.
But that's me. Probably Casey's method is belts and suspenders.