OK. Much has been posted about leather fillets. A search should bring lots of responses.
In short, the leather is supplied in triangular form, just right for fillets. They have been used by patternmakers to construct forms for making cast parts.
I first wet the leather, then glue them in place using Ambroid or SIG cement. While still wet, I burnish them with a round tool to make them concave.
That's all there is. They take dope and paint without any problems.
I find them much easier than messing with epoxy fillets.
Floyd