Third the "aqua fortis" (iron nitrate solution). I make my own by using ferric nitrate crystals dissolved in distilled water or alcohol. I prefer to make my own that way as opposed to dissolving nails in nitric acid because it's a lot safer and I can control he concentration to get the shade I want. You can test a spot, blush it with a heat gun, and wet it with alcohol to see what it will look like finished. If you want darker, soak in more coats or increase concentration. If you hate it, sand it off and start experimenting with alcohol dyes.
Ferric nitrate from the jungle website at 1 tsp/fluid ounce water, two coats, after finishing (ash stock, yours will be different).
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