U might be right with the
Crazy I was smelling nail polish other day , I ask my son 4 years old did u do this he said no , I said u sure he said I sprayed something So u right tht could have done it
WOW
Basically, solvents or extreme heat will reveal whats already there unseen to the eye. If the tank is 15yrs old, these marks most likely were right on/under the surface from the side panel bowing for all that time under load. Again, i personally would not be overly worried. Its pretty much just cosmetic.