I have used an oxalic acid (approx. 10% w/v) solution to remove rust stains
from cotton fabric. Its works almost like magic.

 (can anyone tell me if it actually removes the iron oxide or just changes
the valence state of the iron - ferrous/ferric whatever to an invisible
form?)

Haven't tried it on the pop top, but it is used on surfaces to remove rust
stains, often mixed with a binder like whiting to form a paste.

Caution with this stuff, the fumes are irritating and of course it is
poisonous.

BTW rhubarb leaves have oxalic acid in them, as does sorrel and Skunk
Cabbage (calcium oxalate), and it is the height of rhubarb season now, maybe
rubbing the roof with rhubarb leaves...<g>

Alistair