He has sealed the rust with his tar mixture. If no o2 or h2o can get to the metal it can not oxidize.
That's why I use "oily" type undercoating instead of "rubberized" types (like most asphalt undercoating). The rubberized stuff "dries" then cracks then lets water in, then it creeps underneath and can end up making things worse than if you had just left it.
BUT oily undercoating can attract dust and dirt,.. can seep for months,.. then end up all over you and your passengers clothes.
The tar grease mixture I made has worked quite well,.. It dried out enough to not be so messy but didn't "harden up" like straight roofing tar would have. There is enough grease in the mixture to keep seeping to the metal so that the water/air seal is maintained.
I did go under at one point and the test area underneath and behind the rear bumper was doing well. Some of the mixture came off in big flakes,.. BUT,.. the metal underneath was 'wet",... so I just caked more on.
I also went back to Krown for my yearly spray last April again and that worked out good as well,... the oily spray kinda "re-hydrated" the tar mixture and soaked in.
The 3M stuff mentioned earlier in this thread is kinda a whole different thing. That stuff needs a perfectly clean surface then tries to "bond" to it chemically. If I had to remove all the rust down to bare metal on my car I would be left with just a few plastic moldings and not much else.
On a related topic,... I remember reading that at the time our cars were built (I don't know about now), the metal used to build them was almost 100% recycled metal. All P5's were built entirely in Japan and it was part of their politics to recycle.
Apparently, it is very difficult to remove all the impurities in the reused metal and the remaining impurities (however small) is enough to make our cars "eager" to rust.
I would suggest to anyone,.. especially in the "salt belt",.. to get your car undercoated. I go to Krown (here in Canada) where they drill holes (1 or 2 dozen) then use wands up to 4 feet long to reach inside the hollow spots of the body and frame. The stuff they use is a thin synthetic oil under high pressure and it comes out as a "fog". It covers Everything then seeps and creeps into the cracks and crevices. It takes them 45 minutes to do my car. It was interesting to see that they don't spray behind the side mirrors anymore. Apparently there were a few cars where the mirror glass fell off after the spray soaked in. I got them to spray behind my mirrors again. It keeps the motors working.
I went to a different place years ago where they just sprayed the bottom and were done in less than 10 minutes.
I don't know what kind of choices you Americans have but I would advise to find a place that drills holes.
The holes aren't too bad,.. the have little plugs they pop in and even coat the hole with white grease to help reduce rust.