The acid is long since dried up. The battery burst a long time ago, and I just haven't been able to work on this, but as you can see, I'm forced to take action pronto now.
I know everyone defaults to POR-15 for rust treatment/prevention, but it's pricey, and may not be necessary (for me).
As for the rust remover, I used Rustoleum's stripper on the inside of my driver side fender -- the morons at the body shop who sprayed the replacement fender did a crap job (other guy's insurance paid for it) and it started rusting underneath their paint.
The Rustoleum stripper removed almost all of the rust, after several applications, but I had to use a Dremel wire wheel & magnifying visor to get every last speck prior to priming the bare steel.
I need to remove the fender soon to see how well that primer has held up over the last 5 years. Leaves collect at the fender rear/bottom, and that's where the original rust appeared, so I'm a bit worried.
If no new rust has formed inside the fender, I'll be more inclined to feel confident in just laying down primer in areas the battery vomited on, if I can get every last bit of rust removed first.
If there's new rust in the fender or I can't get all the rust removed from the battery area, then I'll invest in POR-15 or similar.
I think I still have some "rust converter" product around here somewhere, but it's so seldom that I need it that I've forgotten the brand name or whether I still have it (shelf life is probably expired anyway).