Howdy... I don't suggest this as a tried & true method but I had a really good save so I thought I'd share. 19j South bend-thin movement-had a failed hairspring stud-I was unable to get the pin out to repair that one. Ok parts watch. Sadly the hairspring was very slightly rusted. At the main bend for the overcoil & along one side very light. This kills the harmonics & the amplitude was poor at best (150 ish) & not reliable. I metal detect as a hobby & I have used various rust removal methods to restore coins & other dug & crusty items. Not having much to loose-experiment!. Hydrogen peroxide-normal OTC for cuts & things is a great tool. In a small pan heat it slowly on the stove & the rust will start to bubble away. You want very low heat & a pair of magnifier glasses to check the progress. I did mine for about 30 seconds at 1st & another 15 seconds. Then I dunk it in water to kill the reaction. The hairspring will have lost any blue coating in the process & is now very susceptible to rust. Like in minutes. So I dunked it in a bath of oil & cleaned. This seemed to work ok but I wanted to leave something of a coating to protect it without having any residue. I found a soak in Rem-oil & a quick dip in lighter fluid did the trick. You may have other concoctions available but this worked for me. Amplitude is over 300 in all positions now and I was able to get nice steady traces on my timer. In that I'm stubborn-cheap-and almost always over my watch allowance- so I do these things. Hopefully this one works for a HTF hairspring that needs help.