A. Introduction: One of my guiding principles in modeling is that of laziness. Many scale and sport modelers shy away from subjects that have noticeable wing fillets because they think fillets are a terrible chore. It doesn't have to be that way!
Here are 10 steps to easy wing fillets:
1. Make up triangular fillet bases from thin birch ply (I recommend 1/64" for Speed 400 up to say 3 pounds; 1/32" for planes up to 10 pounds or so). The main grain of the ply should be perpendicular to the fuse (across the bases) so that they're strongest on that axis and so that they conform to the wing saddle easily.
2. Sandwich the fillet bases between the wing saddle and wing and then glue them to the fuse (being careful NOT to glue them to the wing!). On a larger model, you can reinforce the joint with balsa triangle stock, but this isn't needed on small models.
3. Remove the wing and glue triangular fillet tails to the main ply fillet bases (these will have a curved trailing edge to have a good appearance). I reinforce the joint between the pieces of ply with a patch of glass cloth.
4. Using a template, pencil the outline of the fillets on the fuse. This will help with shaping the fillets and make them uniform on both sides.
5. Pack a whole lot of light filler into the corner between the fuse and the fillet base. Make no effort at neatness; just mush it in there. You're going to remove a lot of it anyway.
6. Wait at least 2 days, preferably 3. DON'T CHEAT HERE! You want the filler BONE dry.
7. Using coarse sandpaper (80 or 60 grit), sand the fillets to the concave shape you want. Use very little pressure and take gentle strokes; if you sand hard and build up heat, the filler will get gummy, and you'll have a mess on your hands. Only when you're close to final shape should you lightly finish sand with 100 grit. Be a little careful here, as the fillets are rather fragile at this point and will crack if you flex the ply bases downward.