@phil The wikipedia description states that the way stinging nettle works is by injecting that acid using tiny pointy things. This seems to be confirmed by squashed stinging nettle no longer stinging (you can even eat it raw). If I understand that correctly, applying that acid to surface of skin should not sting.

@robryk I read it differently in that it does both mechanical and chemical stinging. I guess the question is how bad formic acid (affectionately called Ameisensäure in German) is in low quantities of water? It's definitely a skin irritant.