@Cyril Jayant -- It's frustrating to see good nibs rust! I really feel for your problem.
I have a few thoughts on the issue, and I'm sure other people will also chime in.
Use: If you are prepping multiples of the same type of nib to put in different holders.... maybe don't? Just use them one at a time, so that they wear out before they rust. At any rate, don't store them in the holders!
Storage: I remove the nib whenever I take a break, clean and dry it thoroughly and carefully, and put it in a box with my "active nibs". When I want to use it again, install it in a holder. When it wears out, I throw it away.
* Note - I ALWAYS take the nib out of the holder, clean it, and store it separately when I'm done, even for an hour or two; I NEVER let it sit in the holder.
* If I think it's going to be weeks or months until I use a nib again, I rub it with a little mineral oil before putting it in a "long term storage" box.
* I keep new nibs in a separate box and only take them out one at a time.
Cleaning Part 1: I'm kind of a neat freak about my tools. I remove nibs, clean and dry them, and store them separate from the holders every time I use them. At least once a week I clean everything (nibs, brushes, ruling pens, etc) in an ultra-sonic bath. Usually plain water is enough, but if I've been using certain types of sumi or india inks or acrylics, I'll use a bit of ammonia to help dissolve the lacquers. Brushes get a restorative shampoo with brush soap.
* If you must remove rust - DO NOT use sandpaper. Soak the nib in WD-40 or Liquid Wrench, then use a soft cloth to wipe off the rust. You'll have to re-prep the nib to remove the residue before you write with it again, but it's fine to leave it on for storage. If that doesn't clear the rust, the nib is done. If you abrade it with sandpaper, the ink will never flow off it quite the same again, and as you're noticing, the microscopic texturing will incline it to more rapid rusting.
Cleaning Part 2: Don't forget to clean the flange! I make sure the flange of the holder is also clean and
dry every time! "Universal" straight holders with internal metal flanges too - make sure there's no moisture in there; those things are huge rust magnets.
INK: If you use certain types of iron gall ink, your nibs are going to rust faster, because the "gall" that keeps the "iron" in suspension is a type of acid. Some brands are more corrosive than others. Diamine's "Registrar's Ink" is one of my favorite iron gall inks (made from the same formula they've been using since the 1800s), but even ONE day of use will visibly discolor a steel nib. After that, they don't last long. They say Fox and Quills inks are buffered so the don't corrode nibs; I long to try them! Some sumi inks are also VERY corrosive - like Yasutomo's highly lacquered KF series ink (green bottle). It's very shiny and very beautiful, but it will eat your nibs. For that reason I prefer Moon Palace (still shiny), and others swear by the matte Yasutomo KY series (red bottle).
Finally - maybe try a different nib? Because ultimately, ALL nibs wear out -- Vintage steel flex nibs are fantastic, but sometimes fragile and often expensive. Modern nibs (steel, bronze, blue, whatever) are cheaper and easier to source. And coated nibs last longer. Nikko and Zebra both make G nibs with a choice of chrome or titanium coatings. They're medium flex, designed for drawing, and often recommended for Spencerian. They're also cheap and plentiful - which keeps me practicing regularly and often, as I'm not trying to "save for best" because they're rare and/or expensive. If you like more flex, there are lots of modern options that give very very good performance, but are "priced for practice" (as I like to think of them).
Maybe you'd be surprised by
how many nibs active calligraphers go through when we're practicing or working often and regularly. I mean, there's a reason you can buy nibs by the gross (and ink by the barrel)...
--yours truly, K