How do you avoid green or gray colors when using aqua fortis (iron nitrate) stain?
Green or gray discoloration with aqua fortis usually happens when the wood is not heated enough after applying the stain. It helps to practice on scrap wood first and to heat until you see a dramatic color change, from greenish gray to orangish brown. The main risk of heating too much is burning corners or charring areas, so care is still needed.