I experimented with a rust stain on black walnut. I mixed old nails and steel wool in a shallow dish, spraying them with a salty solution as needed, and in a week had a good slurry of liquid rust that got poured over cheesecloth.
The “stain” reacts with the tannin in the wood and I really like the results on some dining table legs I made a year ago. In the same room I can see a coffee table I made twenty years ago that had an oil finish and compare its color with the walnut on the new table legs, and, I have to say, I prefer the latter. Also, the rust stain penetrated pretty deep, which enabled me to wet- sand a slurry of varnish, wiping crossgrain to fill the grain a bit.

Best,

Tico