Tamiya XF-21 Flat Base can be mixed with Future to give it a flat finish. Start with about 4 parts Future to 1 part Flat Base and adjust from there. More Flat Base makes it flatter but may also cause it to have a slight white haze. Less Flat Base starts to give a semi-gloss finish.
Presumably you could also mix it into Tamiya gloss acrylics to give them a flat finish as well, however I would be hesitant to use it with other brands of paint without trying it on some scrap first.
It is not intended to be used by itself so it cannot be used for a primer.