I'm pretty sure that all conventional motorcycle fork tubes are chrome plated.
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You might be pretty sure, but I think that you are not correct. Chrome has been used on fork tubes for decades...Zinc and Nickel are the economical option to Chrome Plating and commonly used on machined steel like hydraulic cylinders and steel bolts where additional protection from rust is required. Chromium metal is typically alloyed with the base metal itself to produce stainless steel. Chrome Plating as a process is an expensive highly toxic environmental disaster for manufacturers and chrome layered on the surface of machined steel is prone to peel off in layered chunks like on the brake drum of an old Bultaco. I'm pretty sure based on the typical wear patterns and rock damage repairs that I have done on dozens of motorcycle stanchion tubes over the decades you are looking at mostly Zinc coatings.
First time around the manufacturer was working with fresh new machined steel, the OP is now working with fifty year old worn down and rust pitted steel. Replacement stanchions tubes would be a more economical and likely successful alternative, fork stanchions are not as expensive as the lower fork leg which is even more prone to wear from use. Check the fork legs to see if they are worn oval on the inside as well, because if the aluminum leg forks have no slide bushings or poorly serviced bushings, the lower fork leg eventually develops free play in part of its travel making it impossible to keep good oil seals in them.
The topic is about fork tubes for a 70's Yamaha 650. Don't try to muddy the waters with references to current trials bikes.Alloyed in the steel, not on the steel as a coating. Fork stanchions do not need to be made from really hard material to begin with, trials bikes are all aluminum stanchion tubes now, they ride on nylon coated bushings and a thin layer of oil, Take a file to it or look at the stanchion under a microscope, you won't see layers you'll see worn steel with fine scratches on it.
If he does want to try cleaning up the old ones to sell it, grind/polish the tube in a radial direction not longitudinal, like you would hone a steel cylinder, unlike a chrome bore cylinder which you don't hone.
The Yamaha 650 had chrome plated fork tubes - like EVERY bike in the 70's and onwards.
The OP can get his forks re-plated. There are a few sites that do that.
The cost might be prohibitive, so maybe he should just buy replacement fork tubes.
Might be good to go with some later, more advanced front forks. It depends if you want to keep the original/period look of the bike. Keep us posted.I'm kind of interested in switching entire front ends if it is not too expensive
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