Why bother with the chemicals. Whoever does the metal can just bead blast off all the rust and old bluing (whats left of it), then depending on how bad the pitting is it can then be polished you whatever degree you want. Keep in mind that if pitting is more than a few thou deep it's probably a write off. The pitting can be polished out on a buffing wheel with 140 grit. If there is pitting around any of the numbers or lettering best to leave the pitting or you ususally scrub out any trace of letters along with the pitting (amateur job). If you just want to keep it as is and clean it up and re-blue it, you could just leave minor pitting and re-blue it. Depends on what you expect from something in this condition. If just the barrel is pitted, you can put it on the lathe between centers and take a file to it at high RPM, then work your way down through all the grits to clean up the file marks, I have had to do this many times, doesn't work to well on octagon barrels though. A lot of carful hand filing went into the gun below.
Here is an old Marlin 27S in 25-20 RF I just did, had to do like I said and leave some of the pitting. If you polish it all out you will notice uneven surfaces every time you look down the barrel. Sometimes it is just best to clean it up, blue it and be happy.
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