The cowl flaps were usually kept closed when the aircraft is parked. It prevents FO from getting into them. If needed, they could be pumped open by maintenance to do work in that area.
A 50/50 mixture will not give you the proper color. You need to start with flat black, add gloss black a few drops at a time to get the proper semi gloss black you are looking for.
I would say that studying photos on the P-47’s cowl flaps would be the way to go, Bernie. I like to pose them open, because that’s where they normally are for all ground operations, except long term storage, and in that case the front of the cowl would be tarped and other openings like the exhaust sealed up. As a normal practice during shutdown it is best to leave the airplane set up for the next takeoff, with killer items like trims and cowl flaps in the take-off position.