When my Rapide was rebuilt in the early 90's my Dad took the UFM to work at Dorman Diesels and submerged it in the heated engine cleaning tank.
After 24 hours the paint and rust on the outside of the UFM had gone, and the rubish inside had totally dissolved, including any alloy flakes. An engine borescope was used to inspect inside and it looked like brand new shiny steel. A coat of paint was then quickly applied to the outside and a coating of oil applied inside the UFM as you could visibly see the steel changing colour as it reacted to the moisture in the air.
Not sure what the cleaning fluid was, but everybody who worked there called it "the acid tank". This cleaning tank was used for cleaning newly machined steel and cast iron parts and for also cleaning used diesel engines for rebuilding. Any aluminium placed in the cleaning tank had to be removed quickly as it would dissolve if left too long.
Unfortunately Dormans in Lincoln no longer exists. Pity as at least another 8+ UFM's were also cleaned this way all with the same perfect result.
Cheers
Simon.