I have had success with Sig Koverall. It is a fine product and very durable. I have used two methods to make it stick. #1 is nitrate doping the airframe before applying the Koverall. #2 is using Balsarite the same way. In either case be sure the dope or balsarite is very dry then sand thoroughly. Then comes the important part, removing all the sanding dust. I prefer a small vacuum however a good tack rag will do if applied vigorously.
The first time I learned about filling the surface before painting was the first time I'd used Koverall. I didn't !!! Big mistake. It was on a Kadet Senior and looked pretty good if you didn't get too close.
Fill the surface very well. It should feel super smooth, especially over the framing. After you are sure the entire surface is filled, I use butyrate dope, apply some sanding sealer such as Krylon or Rustoleum, light coats with more sanding. Final sanding should be at least #400 wet-or-dry or finer. Apply your fuel proof finish. Don't hurry the drying process. Too often a beautiful finish is ruined because the final coatings are not allowed to dry well between coats.
The two most important tools for a fine finish are sandpaper and a tack rag or vacuum.