Just my 2 cents worth relating to my hobby situation. I use micro balloons mixed with finish cure epoxy for fillets and filler where needed. Micro balloons weigh next to nothing by themselves. I mix up about 1/4 of epoxy in one of those little graduated mixing cups, and add micro balloons until the mixture is like cake icing, and that just about fills the cup. I spread it un the model with a pop-cycle stick or something similar to get the basic shape. The finish cure epoxy is a 20 minute product, but by adding the micro balloons, it slows the process way down to well over an hour so you have plenty of time to work with it. Lay out and prep the wing joint or canopy for it just like you would for anything else. Once you have the fillets laid in, smoothed out and cleaned up, wait for the stuff to start kicking off and curing by playing with what's left in the cup. When it gets pretty tacky, did your finger in alcohol and smooth everything out and feather the edges down flat. I have never had to sand these type of fillets. A good side benefit is I have never had paint lift off of them either, any kind of paint. I have used this method on wood to wood, wood to covered wings, wood to wings covered with Monokote (overlap the edge about a quarter of an inch) and fared in canopies with it. I'm sure Superfil is a great product, but what I like about the method I use is I always have the finish cure epoxy because I use that for glassing, and other fuel proofing uses, and for applying fibreglass patches to vintage motorcycle parts. I have had other specialty fillet material before that worked well, but it always goes bad before I can use it all because I don't get to build as much as I would like to. This just works well for me and my situation. I think it's a very good alternative to having to send away for the Superfil if you have a hobby shop near you that carries the finish cure epoxy and micro balloons, and one less product to go bad in between uses and take up space on the work bench shelf while getting to that state! What ever you use, if you like it, stick with it because that helps breed consistency. But it's always nice to have a "Plan B".
Type at you later,
Dan McEntee