Just as an addition to Dan's post...
It depends on how strong the NaOH formulation is... if it's stronger leave it on longer. I would always recommend rinsing after the process although. Oxalic acid is a granular based product which when dried re-crystalizes. It can leave your finish appearing blotchy.
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Greg Rentschler
GCR PowerClean
www.gcrpowerclean.com