For applications such as flexible bumper covers, etc. there was a paint additive known as "flattening compound" that we used to kill gloss. I'm not sure if that would work in clearcoat or not. If you wet sand with very fine grit wet/dry paper (1000 or 1500 grit) it will kill the gloss but it might also make the top coat porous. Talk to a professional paint supply jobber- - - -they will probably have a good solution for your situation.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!