Originally Posted by lubloi

Clear coats do not protect the frame from rust the same way paint does, primarily because there is no primer. Salty water will seep through microcracks in the clear coat and cause 'rust snakes' under the clear coat (like what you can see in the third picture). The rust will expand the cracks, letting more water and salt in, accelerating the process. What happened to this Brompton is exactly what you would expect to happen to any clear-coated steel bike in a country that uses road salt. I don't know what Brompton were thinking selling the clear coat as an option in the Netherlands - it was a really bad idea.
If I were you, I would take the bike apart, get it sand blasted, and have it powdercoated. Alternatively, you could sand the frame down using sand paper, and give it a rattle can paint job (two coats of zinc-based primer, then paint, then clear coat). After this, the bike will be good for a few more decades of service.