I base this on a hole i found on the Dolly scuttle. Couldn't work out where water in the footwell was coming from, no bubbles, obvious signs of corrosion. So ground off the paint where I thought it might be getting in and noticed a black patch next to the windscreen surround panel. Hit it with a hammer, dirty great hole.
Had a similar thing on a Rover 75 sill. Thought I had got rid of all the rot, only the welder wouldn't arc. So hit that with a hammer and managed to knock off a huge amount of black material. The whole panel had turned to black oxide. What does this have to do with rust converters?
Well rust converters work by turning red oxide into supposedly more stable black oxide, which forms in a low oxygen environment. The problem is, I have seen more than one panel where black rust has perforated the whole panel and left a hole. It looks solid, but hit it hard enough or prod it hard enough, it falls to pieces.
This makes me suspicious about the effectiveness of rust converters. Sure they may well stabilise the paint work and prevent unsightly bubbles but that doesn't mean that the metal underneath isn't slowly being converted into black oxide.
There is more than one type of black oxide!
iron oxide is rust. Can be red, can be clack as there are a couple of types (chemically)
rust converters react with iron oxide to form Iron Phosphate. Totally different stuff. And pretty stable. Obviosuly will need painting etc, but rust converters do work.
As an aside, they are not miracle workers. As much rust should be removed by mechanical means first, leaving as clean a surface as possible. Then apply the converter to do the last tiny bit. It won't work on rust scabs/scale/flakes etc.