Rust converting

Rust converter paint arrived today. Just tried a little patch. Did a bit more reading. The ones with tannic acid seem to be recognised as the best, and that's what I bought, so phew...

This looks like the fuel line picture I know, but this area was easily accessible and gives a good view of what's happening.

The black areas are what I expected from the write ups. Essentially the surface rust is converted and seals what's below.

Paint (or underseal or whatever) over the top for a protected finish.

The brown areas I am confused by, but maybe hasn't gone off fully, or too thin a coat perhaps. Will be double checking on that.

I have a litre of the stuff to start with and will start with the underside of the car, just gradually work through it. She's going to be outside a lot and needs protecting!!