What does tamiya flat base do? Do you mix it w/ a gloss paint to make it flat, use it like flat coat or use it as a sort of base coat/primer? I tried to use it as a flat coat using a brush but it turned white as soon as it dried & when I touched it, the white lifted off the plastic to my finger. The white kinda feels like fine chalk powder. After the white lifted off, only a teeny bit of flat coating was left. I don't know if this is normal since many say that they use tamiya flat base as flat coat.

Tamiya XF-21 Flat Base is an additive to make gloss colors flat. It can be added in varying amounts to vary the degree of the shine or "flatness" as it were.