I've done the quarters on two Imprezas now. Both with 100% rust removed (even the large amount under the bumper that no one ever sees) and thw wholes welded up with heavy gauged sheet metal. The first one has started to "shine" thru again. The welds and new metal started to rust thru again. The second one was a lot smoother. No signs of any repairs.
I'll be doing 2 more this spring. I've developed a technique so there are no rises or flares where the repairs were made. New metal welded from the inside. Minimal bondo feathering, half the grinding, half the welding. And I have a source for galvanized steel.
In the mean time hitting those crusty spots with PB Blaster stops the rusties from advancing.