A lot of water changes, and keep up a good schedule of it afterwards or itll come back.They clear up but you can usually still see small pin picks where they once were.
How well the scar tissue heals depends a lot on how large/deep the necrosis is, age of fish, overall health of fish, etc. Sometimes they bounce right back, some fish wear the scars for life. Clean water, a nutrient rich diet, and a relatively stress free environment will be your best bet for a successful recovery. Good luck.