Also, you may want to check out any of the acrylic varnishes made by Windsor & Newton, Liquitex or Golden (the three major artist acrylic makers). They should be available at an art supply store. They may seem a little pricey, but drop for drop they are much, much cheaper than the hobby paint brands that come in tiny bottles. You can also mix them with future to get a variety of finishes from gloss to matte.
Also, I don't believe the tamiya flat base is meant to be used as a dull coat on it's own. I've heard of it clouding if sprayed on directly. As MusicCity outlined, you can mix it with future.
Here's a link about it:
http://www.fichtenfoo.com/02GiantRobots/02c-future.html
And another:
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=50206