The original battery had a vent tube which ran down the fender and allowed the acid fumes to escape. They were almost never replaced when the batteries were. You see the results. Use an AGM battery to avoid future problems.

What you have isn't uncommon and looks worse than it is.

First; Clean the area THOROUGHLY. Use lots of soapy water, a scrub brush and rinse well.

Second; Clean the area again, rinse and allow to dry.

Now you can inspect the extent of the damaged area. Don't just slap on some POR15. The odds are you will be merely covering over rust and creating future problems.

Wire brushes and a shop-vac are your friends. While you are cleaning this area vacuum out the hinge pocket area. You'll be dismayed at the amount of crap in those areas.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now