Ok so I used some molasses with water to remove rust from some parts recently and it was working really well to begin with and then the parts got this black oil film and look about them at about the 4th week mark.

Some parts came up fine but others are no covered in black film and the metal is like stained black and won't come off. Anyone had this happen before?

The parts that are the worst are the cams. The lobes of the cams are black as black yet the metal in between the lobes is fine.

All parts were very throughly cleaned prior to molasses.

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Bluey-1973 Clubby - 1330, high lift, big cam, 7 port madness in progress..

Gumby-1978 Minivan, British Racing Green - 1310, high lift, mild cam, enlarged porting and chamber shape with big valve head, supercharged build in ever slow progress!

What ratio were you using? did you stir the mix on occasions to keep from all sludging to the bottom of the tank? were the items fully submerged? was your tank you used clean to begin with was it covered to stop contaminants? was there any alloys in the bath?
My Father and I have had a molasses bath going for about 4 months now and the worst we need do is pressure clean the stubborn tide marks which are normal if we cant submerge all the metal in one hit.
We kept the bath under cover and away from too much sun to stop too much evaporation of the mix, other than that we are using a food grade molasses at about 5 to 1 ratio.

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1968 Morris Mini Traveller - Woody (under restoration)

I used a 1:3 mix to do a block, I stirred it weekly. After 3 weeks I hosed it off and put it back in for another 3 weeks.
Got no black stuff on it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Just a thought,and it will not do any harm if it does not work
I would soak it in a bucket containing hot water and a good amount of
Napisan it should act as a good saponification agent, so I think It may work

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" A Hundred Years From Now It will not Matter what my Bank Acount Was,
The Sort Of House I Lived in, Or The Kind Of Car I drove ... But The World May Be A Better Place
Because I was Once Important In The Life Of A Child"

Yep, the vanish will make it disappear !

my crank stayed in the hot tank at the engine machinists one time and it stained black a bit - may be a similar metallurgical effect you got from the molasses.

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did I tell you that I won a trophy?

Vanish, vanish's varnish

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"Show me the Mini!"

I soaked all the internals of a 4-spd rod change box, and it too came out black. I don't think it'll cause any harm? I haven't put it all back together, but it doesn't worry me. At least the rust is gone!

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1998 Rover Mk VII
2001 Honda CBR 929RR (street)
2014 BMW S1000RR (track)

I did a primary gear that had maybe 30 years rust on it. Seal surface was A bit black. Wire brush and then some 180 wet and dry took it all off.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.