I called a Wood Finish company yesterday to check on the use Oxalic Acid an talked with one of the tech staff who told me they supply P & O with 25kg tubs of oxalic crystals specifically for where passengers spill drinks on teaks, and for taking out water stains.
For internal use he recommended using the crystals disolved in water, and then applied part oxalic and part meths. Once cleaned, wipe again with meths to neutralise.
http://www.agwoodcare.co.uk/index.php
Contact 00 44 1789 778 628<a></a>
Very helpful indeed.