You definitely want to isolate the bit that is affected with the stress mark. I did it with my botcon clench, the foot also had a stress mark. I started off on the highest temperature and about 1-2 cm away from the hair dryer for only a few seconds. If it has not faded or improved in ~5 seconds, than i would stop entirely as it maybe closer to being a crack than a simple stress mark. BUT YEAH, YOU DO THIS AT YOUR OWN RISK!!
I would advise against the use of a lighter, hair dryer only.