The front of the front element of a lens (Tamron SP 35-210/3.5-4.2 BBAR MC, made 1983-1987) has streaks that are visible only when light is shone through them at some angles. They do not appear to be scratches. The element looks fine in normal lighting.
They look like cleaning or water marks, but I cannot remove them. I've tried peroxide, vinegar, 70% isopropyl alcohol, 90% isopropyl alcohol, detergent, water. I left each solution on the lens for about a minute each.
What are these streaks? Could they have been caused by improper cleaning by the lens' previous owner? How can they be removed?
For those interested, I am unable to discern any effect that may be caused by the streaks. Sharpness, color, and contrast are good. There is no noticeable veiling glare when pointed toward large windows, which affects most other lenses.