Last summer I had a beast of a cold sore. It was 6 in one!! I hadn't had one in about 15 years so it was very weird. Probably from vaccine shedding from other people.
For some reason I didn't think to check on here until about day 5. I tried acetone but the skin was already broken and it made it 10 times worse and left a scar too.
I think acetone must be used at the first signs and would be very effective but not once the skin has broken.