Here’s the key to making Flat base Tamiya X-21 work as a matting agent:
Straight from the bottle or mixed with thinner it doesn’t work as except as a frosting agent.
There’s a special technique to getting it to work as desired.
@Mead93 is on the mark, Tamiya x-21 flat base will work as a matt agent when used with Future as described.
@SSGToms Matt’s acrylic flat mix of 30% Flat base Tamiya X-21 & 70% Future" work’s well to matt the gloss issues.
Matt’s Proprietary acrylic flat with - see post #23
Hopefully, Matt will see this and comment in detail too.