Sword polishers in Japan reportedly use a bit of bicarbonate of soda in the water to prevent any tarnishing while they're finishing out. I've done the same polishing knives and it works.

Alex sent me a sample of Shaklee Basic H which is a non-foaming detergent/surfactant to try on JNats and I like it. It has a bit of thickness to the film and washes away cleanly. I don't know if the pH is safe for synthetics like the Shapton Pro though.

Cheers, Steve