Sorry I'm a bit late to the party, but if your fix doesn't work, a good way to blot out any staining that's leeching though (if that's what it is) is a coat of black.
I know that's not an ideal base for your colour but I guess you could mask off the rudder and just black that, then once painted blag it off as tonal variation lol.
Well, I'm going to call this finished.
Certainly not my most enjoyable build, but didn't come out too badly. More photo in the finished aircraft thread.
Thanks to all who commented, without the encouragement I'd have given up.
Pete