What is the best way, apart from using fine sandpaper, to remove slight rust traces from a set of Binci reeds caused by exposure for about 4 years to the Louisiana humid climate. F.i. are there any solvents to soak them in, capable of reaching difficult edges?

You don't want to use solvents. In fact, unless you're experienced in tuning , I would suggest that you leave that task to someone who is. Once the cleaning is done, there is no doubt that some of the reeds will need to be retuned.
Jude

Jude, thanks for your answer. I am experienced in tuning, waxing. A feller into metal suggested to try WD40, and then clean the reeds & plates in thinner. But i'm afraid that there's a chemical reaction then leaving some slight residue behind. Also I had a tip to ask around with antique clock restorers. Another suggestion was use a copper brush, as used for cleaning spark plugs, for the corners etc. The rust is only very slight, and only two high octave reeds are partly stuck, probably by rust traces.
Do you happen to know what Binci uses for glue to attach leathers? Shellack?

Nowadays most people use a Cyanoacrylate adhesive like "Crazy glue".

The outside of the reed can easily be cleaned with a wire brush, steel, brass or copper. The inside of the reed is much harder to clean.

I slip a piece of paper under the reed before cleaning the top. because it is really easy to get a brush bristle stuck in the reed and get it out of position.

Always brush down the reed from rivit to end, not sideways or from end to rivit.

As Jude pointed out, rust on the reeds will change the tuning. I have had a set of reeds that sounded badly out of tune. After cleaning off the rust, it was almost back in tune.

DP

Thanks DP, again Jude. Eddy Grant sang "Gimme hope, Jo'anna", you guys certainly did just that ..hb