I was going to use the flattening agent too. Discussed it w the guys at Harbor Marine and e-mailed Interlux as well. They claim the flattening agent does not affect the performance of the paint. But I'm sure it will affect the ability of the surface coated to stay clean and be cleaned. My issue is w outside decks so if the "bright", gloss and traction issue becomes too much (w the Brightside we are using) we can overcoat w Interdeck that not only has anti-skid but a flatter finnish as well. We may do that anyway as the newly painted deck dosn't offer quite as much traction as the old deck. We mixed 3 colors for the deck paint to get the color we wanted so we will probably need to add the flattener and the anti-skid to our own mix. We call it "three Can Tan".
The deck looks funny in the pic as it's wet.
North Western Washington State USA