I bought some TCS (acid used for chemical peels) off ebay and used it on several skin tags diluted to 50%. Even diluted by half, it burned instantly and the instructions (for skin tags) require multiple coats to be left on for at least 4 minutes. I felt much like Edward Norton during the chemical burn scene in Fight Club. It leaves the area white which then scabs over and peels off. Smaller skin tags required 1 treatment. Two of the bigger tags required 3 treatments.
I have no idea how it would work on scars, but if you're using it over a large area, you're probably better off pain wise to go to a dermatologist. Be careful in the gym afterwords as open wounds are subject to staph infections. Neosporin is a must.