I had the same concern with the rudder on the WNW SE-5a.
In the event I prepped it by spraying it gloss white.
It was left alone to cure for several years ( yes several years) not by intent but because I bought the kit when it was released and finally found time and motivation to do more work on the kit.
I bought some Mr Mark Setter and Mr Mark Softer from a model store in Japan.
The Mr Mark Setter is cloudy in colour and acts as an adhesive to help the decal conform.
I did one side at ea time and then very gently brushed a drop or so of Mr Mark Softer ( the clear fluid) around the edge of the oversize decal. I left it well alone overnight.
The effect was remarkable as the decal wrapped itself around the edge of the rudder and looked as though it was painted on.
I did the same with the other side being careful to align the white areas correctly.

Now just going through the same exercise with my DH-9 rudder see picture for comparison.
Also did the same thing on my Snipe rudder and got the same good results.
Hope this info is of some help?