Thinners /gunke and a reasonably stiff-ish paint type brush ,or,if u can load in car ,take to local garage/lgv w/shop and (usually on a Sat)for £10 get to jet wash it .Or ?do boy scouts still do bob-a-job?
I'd go at it with a small brass wire brush (I get them from the pound shop), or mebbe a brass brush wheel in a drill. Should be ok. Be aware that as soon as you've finished, it'll start oxidising again...
Cif bathroom foam and a jet wash then as snotty said a small brass wire brush I used wire brushes on the dremmel and painted mine afterwards
Looks awesome! Love the red plug on the side. Was hoping to spray my tinware while the engine is out for valve work but I've been told we need to van in 2 weeks to go visit my folks (the missus!). Guess it'll have to wait!