Ahhh, the rust cancer.... the all too common on old cars kept running forever....
Several ways to approach it, you can slow it down, but eliminate? Not happening....
Silverbullet suggested a couple ways that are good, I've done that.
Another way is a grind down and break though so there's a hole. Then on the back side, grind it clean and add a layer of fibreglass/resin.
(I do like fiberglass, tough stuff.)
Then bondo needs to be added and a ton of sanding and finally the painting....
Come back on the inter fenderwell again with woolwax....
This procedure will hold it down for a few years. But round 2 is still in the future...
Alternatively if I dont care what it looks like, I do another light grind off and brush on Permatex Extend rust cure product over it. Let cure overnite and
add a second coat the next day. Sometimes I would also cut and fit in a fiberglass bandaid over the grind off area. And again, on the inter fender well coat the hooey out of it with again woolwax.
Seems like I have had too many rust bucket cars and this is how I have dealt with them.....
For the moment, I am glad to have a generally rust free LeSabre at the moment and did the underbody woolwax thing, and hit the doors with Fluid Film to keep
it that way for a few years..... (Probably sell 2027/28)