Mixed properly Bondo's working time is maybe 10 minutes or so. I think that you would have more than enough time to apply the bondo around the fins. Most people use too much of the hardener and it goes off much faster.What is Bondo's working time? Can you mix small batches of it that will stay usable long enough to enable you to apply it around all of your fins? If the pot life is too brief, then can you just mix up small batches of it so that you can do one or two fillets without having a large amount go to waste? And is it possible to use both? That is, mix microballoons into the Bondo to lower its density and make it lighter in weight?