My flat coat of choice is Pollyscale Flat.
Tweety, you can get Pollyscale here, ask at your local for them to order direct from J&B wholesalers here in Perth. I order the stuff on a regular basis and it only takes a couple of weeks to come in.
Another choice you could well try is Vallejo Flat Clear. Vallejo paints brush paint with no brush marks, so that might be an option. I have seen the results of Vallejo acrylic clear when airbrushed and it is an impressive finish but cannot get any currently to try myself, either airbrushed or brush painted. But being Vallejo, it might be the way to go for you if you are brush painting. Just thin with distilled water.
cheers
Mike