Henna also imparts a temporary resin, which can easily be removed with a clarifying baking soda rinse (which is followed by a diluted vinegar rinse of 2tsp vinegar to 2.5 cups of water and a deep moisturising treatment), or sulfate-based shampoo followed by a deep moisturising treatment.

Before You Purchase: The Beginner
Originally Posted by juanab
This will not remove the henna itself--henna bonds to the keratin, which is why it's so difficult or impossible to remove. It just removes the resin coating. The lawsone (dye) is not removed.
3a/b, F, normal porosity

Suave conditioner, LAL gel