I tried that Oxalic acid on a few parts yesterday and I am totally convinced that is the best way to go
I filled a 5 gal. bucket 1/2 way w/ hot water mixed oh about 3 shot glasses of Woodbleach gave the bucket a few twist and within 20 min. the smaller lighter spots were gone . After about an hour the the heavier stuff was gone
Sorry no pics but those who have posted using this method have pics and now I am totally convinced.
I'm talking I am so convinced ad happy w/ the results that this should be my Infomercial for woodbleach i.e. oxalic acid

I just picked up another box of it last week. You know all those rusty allen head bolts that you have with stubborn rust in the hex hole? I'm going to do a bunch of them next.

tried it too on a frameset - it works like nothing else. This stuff makes a lot of bikes out there previously considered "unrestorable" to have definite possibility.