Asking for advice from any anons that have to deal with rust often. '97 EK Civic from Southern Europe and rust is a non-issue here (almost never rains and the last time it snowed here was in the 1960s), but since I saw a rusted battery tray on an otherwise clean car as a whole, I suspected a battery acid spill at some point in its life and wanted to see how it was underneath.

Pic related is is what I found. I still have to extract the two bolts from the battery tray I had to shear the head from, but since I have no experience with rust at all, at which stage do you think this it at, and how fixable is it?