A couple of weeks ago, I had to sand off about 1/4" of the trailing edge of the wingtip so it would match the aileron. This removed a significant amount of the original fillet inside the tip, so today I built the edge back up with a resin/flox mixture. I used a straight length of 1/4" aluminum tubing to not only insert the filler way back into the outboard end of the seam, but also to form a nice smooth bead.
For reference, I used West Systems 105 Resin and 206 Hardener, along with some flox. (The 206 Hardener is the "slow" hardener, which takes quite a while to cure, especially in a 55-60 degree ambient temperature.)