Ya, Tamiya Flat Base is something you mix with either Tamiya clear coat or other paint so you can control how flat you want your finish to be. Never use it strait as is or thinned, it needs to be mixed with something else.
I use about a 4/1 or 5/1 ratio of Tamiya clear to Tamiya base.
Do a forum search on Tamiya Flat Base, this topic has come up a bunch and there are some great guidelines and info on how to use it in the other posts.