Sorry, but I have to inject a different opinion.
I use Epoxolite for my fillets and I put them on before I cover the model. I only use nitrate dope (non shrinking, thinned 50/50) and put two or three coats on the bare wood. Nitrate fills better and sticks better, too. Then I mask and put my fillets on. Then I sand the fillets and put on about 6 or 7 more coats of nitrate. Finally, I cover the model with light silkspan, fill with more coats of nitrate, and finish with Klass Kote epoxy. You can also cover with carbon veil, but it's expensive.
I don't use butyrate unless I have to shrink the covering and I don't like it very much. As Ty mentioned, it will pull up and bubble. I try to stick with nitrate on the fuselage (and fillet areas) and only switch to butyrate on the open bays.
My .02,
Ron