Repeating Dave's advice...DO NOT USE TAMIYA'S FLAT AS A FINAL COAT. Frankly, I don't know why they even make this.
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A simple flat coat you apply over the paint would make more sense, since mixing it with a base color will give you a flat color you'll have to make glossy again to apply the decals.
Dullcoat in a rattle can will do for you.
Dullcoat is a laquer based product, much "hotter" than enamel or acrylic paint. Brushed over a large surface or decals, it will disolve the colors and make a mess out of your project. You can do minot touch up with a brush, like if you've had to replace a small part with superglue and there's a tiny glossy spot, but beyond that, don't brush this.
If you choose to use a brush applied flat finish, try to find Polly Scale's. Looks white in the bottle, but dries nice and flat.