We just used some flattener on my Sons '29 frame, because we wanted a satin finish. It worked great the second time. We had to scuff it and shoot it again, because it came out glossy. Come to find out from the nice lady at the paint store you have to shoot it sort of dry, because if you shoot it wet it shines, even with a heavy load of flattener in it.

I see there are paints sold on ebay that they tout as flat right out of the can, but haven't used them. When I paint my '39 I plan to use the same flattener we used on the '29.