I had the exact same situation with my Paint in same place: at the heels. I too thought it was scratches, but MTG did not touch it. I finally tried Vertimycin gel: it is WAY expensive $36 for a small bottle, but it works. Within 3 days it was 90% gone! It took another week, 3 times a day, to get rid of it altogether. That was two months ago and it has not returned. I purchased the gel due to the location of the scabs (heel); it stuck really well. You can use it for everything; i even used it on myself for a few big scratches from an unyielding picker bush!!
Enjoy your ride,