I have used battery acid from old car batteries to clean cast iron BMC A
series engine block water passages. Bolted up an old water pump to seal
that apperture and covered the top of the block with vaseline, filled up
the water passages to the top and left it for a day, then flushed out
the whole thing with lots of water. Left it nice and clean. I can't
comment on sodium hydroxide though.

nattydreadlocks wrote:

can anyone tell me if potassium hydroxide is any good at removing
rust? i am going to boil it(mixed with water) at about 200f and use it
to remove rust from a car engine. what do you think?