Tamiya Flat Base

It should. You will at least get a satin finish, but probably not a dead flat finish.

That is precisely why it is made. Tamiya doesn't make a flat clear coat. A little goes a long way.

By the way you should consider using Future as a gloss over coat and Future & Flat Base as a flat clear coat. See Matt Swan's excellent article on The Complete Future

edit it can make a dead flat, as it is an additive you just keep adding the flat base until you get the look you want. In fact you can wreck the paint job by going too far. Don't ask how I know! I would try a 4:1 Gloss to flat base ratio to start with /edit

Indeed, the Future/Tamiya X-21 Flat Base makes a great flat finish.  I used to use Testor's DullCoat, but now use Future/Flat Base exclusively.  When mixed at a ratio of 4 parts Future to 1 part Flat Base, it gives a very flat finish, but does not "frost."  I've used it with ratios as high as 3 parts Future to 1 part Flat Base, but it really doesn't get any flatter, but any areas where the clear coat is a bit heavy, such as inside corners had a slight tendency to "frost."  If you desire a satin of semi-gloss, just experiment with more Future.  The only real caveat is that you need to mix it thoroughly and keep it mixed.

I just tried several experimental batches, trying to get a very slight reduction in gloss. Airbrushed on a piece of cobalt blue glass at 15 psi. Here is what I have so far:

3:1—dead flat
10:1—flat eggshell
20:1—glossy eggshell
25:1—satin
30:1—semigloss
40:1—still semigloss, but glossier
50:1—semigloss, about 25 percent reduction of gloss

I may try a higher dilution, since what I want is to just barely reduce the shine, but I want to see what the last (50:1) dilution looks like in natural light.

Somewhere—don't remember which thread—I read that some had had trouble cleaning Future that had been mixed with Tamiya Flat (X-21) with Windex. So I tested the Tamiya flat for compatibility with Windex using the standard method used by the paint industry to find out.

Tamiya flat is, indeed, incompatible with Windex, whether or not mixed with Future. However, the incompatibility is not severe—as long as you clean promptly and flush thoroughly, you should not have a problem. In the test, it took 24 to 36 hours for the mixtures to flocculate.

But you can still clean it out with mineral spirits and thin with alcohol can't you?

"eizzle": But you can still clean it out with mineral spirits and thin with alcohol can't you?

What!!!??

Clean both Future and Tamiya flat base with Windex or equivalent. Just don't let a mixture of flat base and Windex sit in your airbrush.

Or are you asking how to clean out the gummy mess when you forget?

Future and Tamiya can both be thinned with 90% isopropyl alcohol.