I have black henna stains on a door and have no idea how to remove them. I've tried some alcohol-based cleaners but they don't help. Water, soap and patience while scrubbing doesn't help either and I think I would rather scrub off the paint than remove the stain. Chlorine also seems to have more of an effect on the paint itself than on the henna stain. How do you remove henna without removing the paint underneath?
You can try the following methods:
Lemon juice: Mix equal parts lemon juice and baking soda to form a paste. Apply the paste to the henna stain and let it sit for 10-15 minutes. Wipe it away with a damp cloth.
Hydrogen peroxide: Mix equal parts hydrogen peroxide and baking soda to form a paste. Apply the paste to the henna stain and let it sit for 10-15 minutes. Wipe it away with a damp cloth.
Nail polish remover: If the door is made of a material that can withstand it, you can try using nail polish remover. Dab a cotton ball or cloth in nail polish remover and rub it onto the henna stain. Wait a few minutes, then wipe it away with a clean cloth.
Sandpaper: If the henna stain is particularly stubborn, you can try sanding it away with fine-grit sandpaper.
Note: Before trying any of these methods, test a small, inconspicuous area first to make sure it doesn't cause damage to the door material.
I'm afraid the best solution will be to paint over the stain. That seems to be the most pragmatical option and the one that probably works best.
Henna is not a paint in the sense that it leaves colored pigments (that can be washed away), instead it is a tanning agent that changes the color of the material it touches. If it touches human skin, it darkens the skin itself, so you can wash the henna off and it leaves darker lines behind.
I don't know what type of paint is used on your door. If it's not water-repellent, then the henna might have soaked into the paint or even the wood and darkened it. If you're lucky, the henna only darkened the top layer of paint and you can sand it off.
Maybe a repeated application of a bleaching agent can lighten up the stain again, but from experience the color will never return to the original color. The henna changed the chemical composition of the paint, but bleach cannot undo this change.