Another idea to shape this stuff for the fillets other than the use of a finger --
Depending on the radius desired for the fillets, you can use the sockets from your ratchet wrench set. For smaller radius fillets, you can use a length of tubing or even wire. Smear the Super Fill around the root (or wherever you are applying it). Use whatever you select to form the radius (even your finger as suggested in a previous thread), dip it in alcohol or thinner and draw the item around the fillet. (Water even works for this but I think alcohol or thinner is better.) You might need to do this several times to get the fillet completely uniform. Then you can take a pallet knife and scrape off the surplus left by the "radius forming tool". For me, the working life of the Super Fill is good to complete one fillet, top and bottom on one side of the fuselage. Done right, the fillet is smoothly formed without any more shaping, then all that is needed to do before continuing the finish process is to sand the surface of the fillets so they are not shiny to allow the paint to adhere to them.
And you can clean your "radius forming tool" with that alcohol or thinner.
Keith