Help! Can't primer backside of metal body patches - worried about rust

My neighbor's car ('56 Chevy) has a couple of areas where the the previous bodywork was a bit of a bondo BS job. As a result, after a few years the paint has bubbled up in a these places. I am planning on creating some sheet metal patches and welding them in place. They are located on the top of the rear quarter panel. After I complete the metal work, my plan is to spray some basic auto primer on the patches to keep them from rusting until he figures out whether he's going to re-paint the entire car or just the patched areas.

My concern is that it doesn't look like there's any easy way to get access to the inside of the quarter panel. This means that the metal patch will not have any primer on the inside, and I'm concerned about rust developing there. Is this a legitimate concern? Does anyone have any experience on how to handle situations like this?

Thanks,
PeterM

I use weldable primer on the inside of panels I know I will not be able to get at with the paint gun.

Yeah the 3M stuff works real well. But primer aint waterproof and it can cover up rusting.