For field work where we didn't have tannic acid handy we brew up a six bagger pot of tea and steep to the point of blackness. The resulting brew did quite nicely for preserving archaeological iron artefacts.

I would recommend loose tea as the tea bag often contributes to the brew. Unless you really want a staining developer I would use distilled water or at least reverse osmosis water and not well water for instance. Even city/house water can have appreciable lime and ferrous ions as well as curpous.

Regards

Mark MacKenzie