Now on to injected fillets. Normally I would just skip over this step, but there was a thread a little while back where the poster complained about how onerous the process was. I'm not a real fan of this technique and always attempt to get access to internal fillets via either end, but when needed, I do use it, and after stubbing my toe a time or two, I have it down to a fairly painless process. The preamble to this post is: 1) make sure your fins fit fairly snuggly against the CR's and the root edge is flat, 2) whatever adhesive you use, make sure it is slathered on the root edge and each fin end, aft and fore, liberally with fillers so that few, if any gaps exist, 3) to guard against any small seepage do the preliminary gap-filling fillets shown in post the previous post.
Gather your materials... Besides a syringe, there are a couple of key items here. Use low viscosity epoxy like U.S. Composites or West. I have found that about 1 gram per inch is just about the right amount for internal fillets. Crazy Jim has shown doing this procedure with chopped carbon. I have never been able to get this to work (same for chopped FG) so I just use milled (1/32") FG and fill the epoxy until I get a runny slurry. The other key I have found is to have "crisp" holes. Whatever airframe material I use I put a drop of CA and a spritz of CA hardener in each hole and re-drill until there are no burrs/fibers in an FG airframe or scraps in a phenolic or paper (as in this build).
Make sure your airframe is exactly horizontal. Again critical.
Don't skimp on holes (I have four spaced across the 20" root edge). More importantly, make them big enough. I mostly messed up when I started using this method by making my holes too small and too close to the fins. This combo tends to make it easy to plug the holes when you inject the epoxy and then it backs up on you. I now always make them nice and big so I can get the syringe fully in the hole with ease. I also pull the holes away from the fin so the epoxy drops vertically into the valley of the fin and airframe (note how crisp the holes are after CA application and re-drilling).
After that inject the epoxy slurry freely (again with about 1 gram per linear inch of root edge). After injection then mix up some 5 minute epoxy with a very healthy amount of microspheres. This thick mixture is then used to plug the holes in the airframe. Push it in the hole with an applicator (popsicle stick). It will probably take two applications in succession as the 5-minute epoxy begins to set. Wipe all overage of 5-minute epoxy off with alcohol. You probably will still have some cupping/weeping as the epoxy sets in the holes. Don't worry about it as the next step takes care of it.
Final step is to mix some mid-cure epoxy with some filler for fillets or just some milled FG. I use either Bob Smith mid-cure or System Three T-88. Both tend to be high viscosity and will set fairly quickly and run only minimally. Use this to make your external/aesthetic/aerodynamic fillet shape. When making these fillets, simply use your applicator to "inch up" and fill in the cupped injection fillet "divots". This method is much better than having the holes close to the fin while you try to apply an external fillet as the holes inside the fillet area will result in uneven fillets.