The Spies Hecker matting paste that LCW mentioned is: 'MA100 matting component'
I have used it on some bike stuff and also on helmets if thats what people want. It retails around $ 109 a litre and its mixed at 1:1 for matt and less for satin. I will try a new product by Glasurit soon. Its a clearcoat that just needs hardener for a final coat over whatever colour base you have, It comes in either matt or satin.