Layering up any composite requires the first layer to be well roughed up with 60 grit (or more coarse) for good adhesion. But, do you really want to take a large lump of filler (any filler) flying? Perhaps remodel with polyester filler (easier to sand), cover with glass and grind out the filler. Whatever you do I would apply glass over the filler to keep it all together in the long term.