Acetone, which you can buy from a boat store that deals in fibreglass boat, building materials. This should, with a swift wipe, remove the surface gloss, leaving the finish dull. Don't scrub, don't overwet the cloth, keep using a new bit of the cloth, as it may clog up with the stuff you're wiping off. Put it on the cloth, not on the work, as it's very runny and can't be controlled in direction or effect.
Lastly, wear a face mask, and do it either outside, or in a very well ventilated room.
Acetone is highly flammable, and volatile, but benign if handled correctly.
Test a small corner first of course, just to make sure it will do what you require.