So some good news / bad news:
The rust in the trunk from earlier was just all this added seam sealer from the previous owner's repair that swelled up and started breaking through the paint. Solid metal underneath! Cleaned up all shiny like my deadpedal bracket.
The bad news is that
right beside this repair, there's a small hole.
It's lasted 20 years like this on it's own, so for now I filled it with JB Weld and slathered on the the rubber sealant. My bandaid patch job should hold until I'm at the paint & body step.