Midway suggests a 3-coat layer with 15 minutes' drying time between coats before curing in a 350 degree oven for an hour, which adds about .005" in my experience. It's enough that they don't recommend using it on gun parts that have a slip fit like pump action shotgun slides or lever action operating levers. It would probably be necessary to polish the piston bore for a smooth slip fit after coating the piston. I'd use a piece of slotted wooden dowel rod with a piece of 600 grit wet/dry sandpaper wrapped around it, and a cordless drill to fit the hole to the coated piston.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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