For some reason some of the pictures do not appear. I'll add them below in another post
I like fillets made from micro-balloons mixed with 30-minute epoxy. They've held up well without any dope lifting from them or any crash damage in multiple earthly encounters. I've posted some of my techniques before, but here's the current project, showing most steps.
1) This is the amount I mixed for one 11”+ wing fillet. There was too much left over this time. I used a piece of wood or a tongue depressor to mix this thoroughly. I spread it out on foil to slow the set-up time. When I use a cup, I have to use the graduation marks and when the epoxy is mixed, I pour it onto a foil-lined paper plate anyway.
2) After mixing the epoxy, I pour on a large amount of micro-balloons and mix in. I do this repeatedly, until no more will mix in. It absorbs an unexpected amount of these to get that cake-frosting consistency mentioned above.
3) This is one of the roots to be filletted. If there is more gap, the slurry will help fill it at some weight gain.
4) Here I tape off a root area before applying the epoxy/micro-balloon mix. Space depends on your choice of fillet radius.
5) This is the mix, and these are applicators. I either use the radius given or sand the tip to a different radius.
6) Here is the fillet material just after application (I didn’t take a picture during application, because I was , uh, busy). While it’s still wet, I pull the tape off and scrape off any splatter with the edge of a #11 exacto blade.
7) This is the initial result.
8 ) After removing the tape, I fine-tune the shape, not expecting it to fully retain this curve. However, I want the width and radius to remain as constant as possible. This is also the chance to feather the edges, where the thickness of the masking tape raised the edge thickness of the fillet. I have to be careful here not to do this too soon, but not too late either, since the stick can lift or rough up the material. Some use water, fingers, or acetone to help shape later. Wet fingers for me have not given the uniform thickness I want.
9) When one fillet is pre-contoured as shown, I brace the plane in this position, with the new fillet at the lowest point, facing downward. That way the fillet material does not puccle in the middle and loose its width.
10) After a few hours the fillet may be sanded. This is the initial sanding (using appropriate size dowel wrapped in, say, #320 wet emery paper and with Dupont PrepSol. Dry Garnet paper of higher roughness may also be used, as I did on this particular one. I took this picture to show that my fillet lost some of its uniformity while setting. I sand the fillet until it is this light color and uniformly smooth, being careful to protect the doped adjacent surfaces. I often re-apply masking tape along the edges and sand into it when finishing, to feather the edges. I’m often less successful than I’d like and have to, if it’s worth it, fill the edge areas with white primer (Brodak) or some other sanding sealer. My skills have apparently diminished. Anyway, the surface is then free of the waxiness and rough enough to help dope to adhere.
11) Here’s a photo of initial sanding, with the low spots showing dark.
12) Here’s another way to help ensure that dope doesn’t lift. This picture shows my first clear dope application to the fillet. I use clear first, because it has no pigments to get in the way of adhesion. I only dope up to and including a part not exceeding ¼ the width of the fillet on each side. That way these areas can dry and shrink for a while, before any more dope is applied.. After this a steady-handed modeler can run a narrow, non-overlapping bit down the center. If not, then just apply to one edge, with minimum overlap and let dry for an hour, before doing the same to the other edge. This way the dope shrinks in small areas of minimum curvature. After this, I put still highly thinned (50/50+) dope down the center with overlap. I repeat this procedure, until I have a sheen and then wet sand with , say, #380-400 just to roughen the surface for pigmented dope or...whatever.
13) Here’s another initial application.
14 –16) Last three. show fillets after a brushed and sprayed coat of gold underlay. I agree about spraying, ESPECIALLY since I use the PreVol spray cannisters, which spray a narrow band, not too consistently; don’t spray directly at the fillet, unless you have a deft touch or a very fine spray. I brushed mine on first in places, using the partial coverage technique mentioned above. Then I sprayed, letting the edge of the spray area overlap the fillet. I also sprayed from one side of the fillet and went on to other fillets or parts of the plane, before returning to spray from the other side. After a day or so, I sprayed clear over the entire plane so that the next coat would have the best possible adhesion.
I PRESUME that this application will be as successful as the others.
SK