ORIGINAL: Jim Clause
I have spent the last two days trying to get the transition between the vertical fin and the fuselage to look decent.
He, he... fun isn't it!?
Jim, I'm not sure how you're going about it but masking tape is your friend. The fillets in the pictures above look plenty meaty which is good. I would mask surrounding areas that don't have filler to avoid gouging the wood, then sand them down a fair bit until most of the filler is gone except for a 1/8 - 3/16" radius. It can be rough since you'll build it up to ~ 1/4" to 3/8" radius next.
Once the wood is clear and smooth, remove your masking tape and reapply one strip carefully on the top and one on the bottom curving them around the LE to meet at ~1/4" in front of the stab root LE. Likewise, the masking will be separated from the stab at the fuse 1/4". Don't worry about the TE as you don't want much of a fillet there to speak of - just something minor. When you prime the fuse, it will fill in nicely at that spot.
Once your prep work is done, you can then proceed to produce the final fillet. It looks like you might be using bondo type material. Try using epoxy and microballoons (the white kind) mixed to a creamy slurry - it takes quite a bit of MB. You will be able to get a better contour with no need for sanding by using this stuff. You lay it down between the masking tape (by the way you also need to mask the stab of course same distance as the fuse) with a mixing stick and then dip your finger in alcohol to smooth it down into a nice contour. Some of the mix might come off on your finger - just wipe it off with a paper towel. Keep your finger clean and wet with alcohol. The mix will absorb some of the alcohol and if your finger gets dry, you'll start to "smear" the mix so keep it wet and keep it running. One single pass across from LE to TE.
I like to do both sides on the bottom or top first as it is easier not to have to flip the fuse and you can make them symmetric by eyeballing them. I use 30 min epoxy. When you're done with the smoothing, you'll probably be ready to remove the masking tape as it takes about 15 minutes to get the contour right. If you don't lift the masking tape when the mix begins to set, you'll end up with a mess but you don't want to do it while the fillets are still wet - let the alcohol evaporate and the mix set a bit. Peel off the masking over itself to avoid lifting the mix. It will leave a nice uniform line and fillet. Also, if you lift at the right time, the mix will "settle" down leaving no ridge at the mix/wood junction.
Once that is all done and the mix is hard (wait a day), remask surrounding areas, take a small piece of 400 grit and sand lightly to remove any "speckles" off the surface. This will provide a good base for primer.
I'm not sure when you painted last but when your model is ready for finishing, make sure that the glass work is nicely filled and sanded to 400 grit. I have been using a sandable base primer to show me the imperfections, followed by bondo and then wet sanded down to 600 grit. Once the dents and pinholes (I have many more of these than you will since my fuse is glass) are taken care of, I am using a high build automotive polyester primer - great stuff. A tad expensive but worth it. 80% of the work is in the prep as always. The actual paint is kind of a "treat" after masking, prepping and sanding for hours... But then, you probably know all this.
David.