Yeah this shouldn’t even be an issue, unless we want the player to be able to apply lye just for kicks as it would do literally nothing. Have you not seen Fight Club:
https://www.youtube.com/watch?v=TfDVLsBXYcM
That is lye, and you want to be able to stand outside in rain (acid or not) with that all over you!? admittedly, the Fight Club scene was a massive exaggeration with that much moisture/time, but 10 minutes plus loads of water…
BACK TO THE TOPIC AT HAND:
Pain is fine, although I’d prefer it to be more graduated and perhaps cause some additional effects (I’m not quite sure what). Pain is pretty easy to remedy with pain killers. You can just wait somewhere safe, chuck some drugs and be on your way. Possibly if there was ‘burnt skin’ or something which had a penalty?