Steel pistons or aluminum? I bought an F20 with aluminum pistons that were oxidized. There wasn't much rust at all but those babies were in there tight. Soaked them with every elixor I've ever heard about for a year and they wouldn't come loose without a good pounding with an oak block the size of the piston and a sledge hammer. I have been successful with a few lightly stuck ones using ATF.
I got lucky one time screwing a hydraulic hose in a spark plug hole and putting hydraulic pressure to it, making sure the valves were closed first. I said I got lucky because I could have bent rods or broken the crank slicker-n-a whistle. One suggestion I heard was to find a piston that's near the top and concoct an adaptor to fit a grease zerk in the spark plug hole. Fill the piston with oil, then start pumping in grease with a grease gun. I can see a bent rod happening that way too.Jim