A character has some cuts. They're not very wide and they're deep enough to scar but there's no reason for the character to rush to the A&E right now or anything. These cuts were inflicted minutes ago, so they're wide open. There happens to be a doctor in the room. Good! Except not good because this doctor is an asshole. The doctor does not want to do any lasting damage to the injured character, but he does want to cause the injured character pain under the pretence of healing him. What is he going to apply to the cut? Assume that, for whatever reason, he has access to literally any substance in the world. Bonus points if it comes liquid form and can be sprayed onto the wound, but that's hardly a requirement.
I need things which could be put on the wound which will cause pain (the more pain, the better) but which are extremely unlikely to actually cause an infection or otherwise seriously hinder the healing process. It's not the end of the world if they slow the healing down a little bit, but nothing that a competent doctor's medical instincts are going to strongly rebel against (e.g. no insecticide or anything). The substance can also help with the healing as long as it's very painful. The main thing is the pain.
Searches: "most painful ways to heal a cut"; "most painful things to put on a cut"; and various specific possibilities (salt water, alcohol, etc.)