New Primer to Original Paint/Undercoating Interface

[Amik]

Hello all, I've recently acquired a 1975 911S and I'm going through a fun process of investigating for rust in the usual suspect areas (thanks for all the posts here which have guided me), and I've managed to find some. Right now I've got the fenders off and have found rust on the body just forward of the top of the "fender joining plate" along with the top of that joining plate being rusted out. I've cut out a small 1"x1.5" section and will weld plate back into that spot and fix the fender joining plate. Also found the very bottom of that fender joining plate is thin and will need a 1"x2" patch. And finally, some rust along the length of the edge where the fender mates up to the body. None of it is concerning me structurally so far, but I do want to make sure I do the metal prep, priming and paint steps correctly to yield a sustainable result.

That brings me to my question. I've removed the undercoating and paint in the rusted areas + an inch or two, but I'm not sure how to make a clean transition to new primer. Do I just need to go to the point where I feel confident the original paint/undercoating is holding up and then get that reasonably clean and allow my new epoxy primer to overlap from the bare metal onto that paint/undercoating surface? Will the transition from bare metal to the original coatings be a vulnerability in the near term?

For the bare metal areas, I'm using Glasurit 360-4 metal cleaner, then Glasurit Epoxy Primer and finally their 22 line paint. My plan is to address all the bare metal, clean/prime/paint, then reinstall seals, butyl, etc... and when it's all back together, spray on new undercoating and then another layer of black paint on the undercoating.

Really just looking to stop the rust progression for now because it's clear this car will need a full paint job at some point, but I'd like to get a larger plan together before tackling that full effort.

Example photo below. Just cleaned so far with some wire brushing to determine rust extent, more to do to get it ready for primer.

[Bill Douglas]

Greasy Fingers did a job a bit like this. Check him out on youtube.

The car looks like it's in really good condition.

[Dpmulvan]

Tape off area where butyl tape goes, shoot epoxy on inner fender chipgaurd area let it flash then shoot chipgaurd. Un tape butyl tape area shoot epoxy let flash then shoot color.

[Amik]

Thanks for the help guys. Still cleaning up the fender itself but hope to get moving on it again this weekend.