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can't be polished up then?
Nope, the dullness is down to the preparation of the substrate and not the actual chroming.
At a guess, batch producers found it cheaper to dip the whole frame, rather than just the "show" parts: just as with paint, prep is everything (and costly).
Again a guess, but an actual "chromovelato" (sp?) finish would require "show" chrome even under the paint, which would explain what @tallsam found under the paint of his Nightburner versus what I found on my Olagnero.
How does the finish between the rear triangle and the revealed chrome on the top tube compare?
The chrome under the paint will be shit and as that's down to surface prep, nothing you can do to improve it.