The Stinson cowling insides were not painted from the factory according to my Stinson expert Gary. I didn’t want to leave mine bare so I primed them and will paint them an aluminum color that can withstand extreme temps, oil, and fuel.
This is a sample coating of the color:
Stinson Station Wagon, NC9391K Restoration 
I agree and think that’s a good idea. Anything you can do to add to the longevity of the airframe, with out messing up the original appearance.
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