Firm

I've got a 96 Bird...nothing fancy, but my fiance and I use it as our primary car because in the 8 years we've had it, it it's proven reliable and still a great low cost versatile daily driver.
We live in the salt belt and the car is winter driven. I do everything in my power to keep the car rust free, and thus far my efforts have been rewarded. The car's body and underside have remained 99.9% rust free.

Except for this one nagging little area, it's on the inside of the rear wheel well, on the inner edge towards the front of the wheel (see photos below to show area). Years ago I noticed a bit of the factory undercoat peeling, so I scraped it off, wire brushed the area (including the back side) to bare clean metal, primed the area with rust inhibiting primer, and then sprayed some fresh undercoating. It was great for a few months and then I noticed some rusty looking color bleeding through in a couple spots....So thinking the primer must not have bonded or something, I repeated the process....Few months later it comes back.

This has been going on for years, and I am getting sick of spending hours every few months grinding and treating this area. I don't think it's my process/treatment....I am not technically a body guy, but I do vintage car restorations as a side gig, so I am somewhat knowledgeable when it comes to dealing with rust.

So, my question is...wth is going on here? How does rust start bleeding through so quickly? Is water somehow getting trapped between the two panels at the pinch weld here??

My car with arrow showing the area:


Photo from installuniversity showing the area I am talking about:


The actual area...you can see a little rust bleeding through:

ZexGX

iTrader: (28)

It's possible that there are holes/gaps. Another thing to check is for leaks/holes anywhere else in that area of the car, top to bottom. Check the area around the bump stops - perhaps there is a gap along that seam.