More brasso today, moved the car to the workshop, where I'll start the real work.
Spent this afternoon cleaning it a bit more.

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As you can see, the paint is far from good, but the brasso seems to be attacking the worst of it.
I won't get it perfect, but at least I can seal in a fairly decent looking layer once I've got all the crap off.
I guess it's years of tree sap eating away at it that's caused it.
The door will need a repaint, but I reckon I may well get away with just cleaning the rest, it'll never be a show winner, but the idea of this car is a cheap daily driver that I enjoy.