when somebody cuts out say, a rocker panel, or wheel arches over a truck bed side, and replaces with a patch panel, when done they look great//// and when done right, can't be told that anything was ever cut out and replaced..... but a couple years later, the seam itself rusts, and you can tell exactly where the parent metal was cut, and the patch panel lies.... the seams made during this process, then rust. Is this because of dissimilar metal, between the parent metal and the patch? would this be less likely of the patch was cut out of another car like the one being fixed? (if one with good metal where yours is bad, can be found) the metal rusts and bleeds thru the paint, exactly at the seam of the repair.

I suggest dissimilar metal being a cause, because of my experience as a millwright in a steel mill for a few years in my past, and watching what it took to get the mix right// while the ingredients were being melted down and poured.... and today's imported patch panels certainly aren't the same metal composition as the original metal---- but it's all we can do these days, is to use those, if we must fix our old rides........ not much good, original sheet metal is available, any more.

I am not gonna say that its the weld wire in the MIG, that was used at the time..... though it could be..... or the "heat affected zone" because the metal was super heated by the weld process.... because my son's 99 Ram 2500, which was repaired and repainted before he bought it, the replacement wheel arches were "panel bonded" instead of welded, and same result..... though I dunno if a combo of welding and panel bond was used/ since it was done before my son bought it.... can just go by what the PO said when he bought it, and the pile of reciepts he got with the truck, that show (at least some of) the materials used..... but when Panel bond is used, metal isn't heated as it is when panels are welded in. but the problem of seam rust obviously still exists then, too.

I have some ongoing sheet metal work going on my Volare and am going to be starting sheet metal repair on my 85 pickup this summer. I DON'T want this happening to either one down the road, after I put all that work into them.

anything that can be done, to keep this from happening?

Don, I am no more than an apprentice welder, but I have read and what was discussed in the welding class that I attended is for automotive sheet metal body repairs always do butt welds and stay away from overlapping the panels. No matter how tight you think the overlapped panels are, there is space between them that will trap moisture.
The discussion on using weld through primer was those products get mixed reviews. What was agreed upon is to get a good seal to prevent moisture intrusion into pin holes, micro voids ASAP, so finish grind and seal the weld promptly. A modern sealing paint like POR 15 would be my choice on the backside of the weld repair.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Huh. Lap welds are stronger, but I can see what you mean.
I haven't done alot of this sort of repairs but have done some.
For some reason the 1st one I ever did was the only one that didn't do this, and I did it the same exact way.
So I was thinking that back then they used different metal to stamp the patch from.
I'm not just seeing this in my work/ when driving around I see it on other people's vehicles too. People who I do not know.

And getting to the back side of a repaired panel, isn't always possible once it is closed up, new metal in place.

The Fe in steel likes oxygen and those two together make Fe2O3 or what we call rust.
The only way around that is to add enough chromium/nickel to the Fe and make stainless steel.
Even then, all that is happening is the chrome/nickel is making a protective coating around the Fe molecules so the O2 can’t attach to it.
Keeping the Fe and O2 apart is the key.The back side of an enclosed weld is usually accessible by drilling an access hole or holes and then spraying in a material to seal the weld. Eastwood has spray cans with long flex nozzles that will do the inside of car frames. Or similar could be rigged using a pressure pot and a weed sprayer.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Very interesting discussion, as I've often wondered about that rust-on-welds issue.

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)