POR-15 Surprise, "rust never sleeps..." Options

[ss6]

About 2 years ago now, I had the gas tank out during my six conversion and touched up some patches of surface rust on the painted metal surrounding the tank with POR-15. (I used their clear stuff 'cause I wanted to keep an eye on some of my welds elsewhere in the car.) Got in there with a wire wheel first to remove the loose rust, but following the mfr's advice:

"Surface preparation:
Rusted surfaces are best; seasoned metal and sandblasted surfaces are also good. "

So I didn't grind all the way down to shiny metal.

OK, 2 yrs later, tank is out again (chasing down a persistent fumes issue). One glance at the POR'd areas and I see NEW rust "spiders" (throught the clear POR) growing out from the original area. Nothing major, no "bubbles", but stuff that clearly wasn't there when I painted it.

WTF???