Warning: I used to track every spot on my old maps with a red pen, then a librarian in Des Moines showed me what that ink does after 30 years | Collectors of Weird Maps - Topic Shares
Man, that hits home! A buddy of mine did the same thing with red pen all over his hiking maps, and about 15 years later the ink had bled through to the other side and stained the whole back panel pink. Now he just uses pencil and takes a photo with his phone for backup.