To do this rusting you need 3 spray bottles. The size of the bottle, and hence the nozzle, makes a difference to the result so it pays to experiment. First bottle - a concentrated solution made of tea, we used regular gumboot but I will be experimenting with herbal teas later. Second bottle - solution of ferrous sulphate. Third bottle - weak solution of caustic soda. The second two ingredients come from the hardware store, those blokes at Mitre 10 will be looking at me sideways yet again..... "what, a woman wants to buy men's stuff???".
Peg your paper on a rack over the lawn, or surface where drips don't matter. Spray your paper with each of these solutions, starting with the tea, following with the ferrous and lastly the caustic. Don't wait for the layers to dry, drips running down the paper adds to the effect. Drips are your friend. If after the first go your paper isn't quite what you want, then keep going with layers of each solution until you have something gobsmackingly fabulous. Once the paper is dry you can rinse it with plain water to remove the caustic.
Experiment with different papers, we found that the cheap ungessoed book cloth absorbed the liquids better than the one with gesso. Papers that are coloured or printed give wonderful results.
Thank you Zona for sharing your amazing knowledge. Days like this bring nourishment to the soul.