I am looking for a product that can be mixed into gloss paint to remove sheen.?

You need flattner. Most auto paint stores will carry it. DuPont makes a flatner, part # 4531S. You need to be sure to add the proper amount or you could end up with a chalky sheen.

If you want a non shiny sheen you should get that kind of paint. They make all kinds, flat, eggshell,semi gloss, high gloss, and so on get the right kind you want.

You can try talcum powder. Be sure to try this on a scrap piece of metal and see how it works for you. I used to use it when I was building model cars, but the theory should still hold.

good luck

Are you talking about something like a flat black paint job?

Flat Black:

http://derfotohof.net/autos/macde/IMG_0950.jpg

http://farm1.static.flickr.com/172/480321176_8d724...

If so you need that kind of paint.