I put these images in the C&V appraisals forum, but I'm not sure it was noticed. I looked at a Trek 720 today and saw some spots of paint damage and minimal rust, along with these worm-like tunneling shapes in the paint. In the photo below you see one in the middle of the photo and another on the right-hand edge of the photo, both on the chain stay. There was a similar corrosion pattern under the bottom bracket.


It looks like a thick thread was on the frame when it was painted. How worrisome is this? What would I likely see if I sanded the paint off?