POR 15 is excellent stuff and so is Good Stuff, but if you are experiencing
bubbled up rust then it is already too late. as the metal has already been
eaten through. At that point, best to sand down the affected area , removing
all rust then use POR 15 or naval jelly to kill any remaining surface rust.
As there are bubbles, this most likely means that there will be holes left
where you sanded down the bubbled up rust. If there are holes, you will need
to bondo them in and sand the surface smooth again before priming and
painting the surface to match.

There are lots of good how-tos and tips for how to repair rusted thru panels
if you search Google.

marx404