I got Andy hooked on leather! Kinky!!! Anyway, you soak the material in water to soften it up, and stick it down with old fashioned Amberoid or Sigment. I thin the material where it goes around the extreme nose of the airfoil, but then I am doing 1/2A models!. Yes you need a spherical tool to press the fillet into the joint to make it seat firmly and have a flillet-like look. Otherwise, it will just be an angular filler and not bond firmly to the wing and fuselage. I think it is more reliable and lighter than the epoxilite versions, but does have to be done carefully.
It ain't cheap! But I prefer it. How many models do you build in a year, anyway? (Except for the Buttafuco Stunt Team) Freeman Co. is the source of a wide variety of sizes. Do a Google search on Leather Fillet and you will find them.