Jacs,
Have you covered yours? And if you have, did you cover the wing fillet or just paint it?

I'm not too sure what you're referring to as a filling device. If it's a fuel filler, I've been using fuel dots. Dubro makes some nice and simple plastic ones.

As far as a flattenner for paints, you can use talc and a bit of reducer to reduce the gloss of about any paint.