John,

It really depends on the current condition and previous surface treatments (coatings) on the valve.

If there is an intact coating - such as paint - then you want to preserve it as much as possible. Tannic acid will turn the surface black and it will not protect the surface properly if it in on overlying paint or a heavily grimy or corroded surface.

Sandblasting is too aggressive a technique and is not really used in the treatment of outdoor sculpture or industrial artifacts anymore. There are other air abrasive media that can selectively clean large metallic surfaces safely to a stable oxidation layer without "refinishing" the original surface.

Your best bet is to consult a conservator about the condition of the piece and how you propose to exhibit it.

Cheers!
Dave

David Harvey
Conservator
Los Angeles, California  USA

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