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Substitue "Bayan deep bass" reed block into Italian accordion

A great project- likely to be rather challenging but ultimately- even if not completely perfect at the end (and I surely hope it will be absolutely perfect)- enjoyable.

"The noblest pleasure is the joy of understanding." L da Vinci

Tinkering surely fits the bill.

PS FWIW I'd consider using brass brushes vice steel after the soaking for the reeds. Steel brushes will surely have the potential to scratch things up on the reeds (plates AND toungues) which might well cause annoying secondary effects. Brass brushes would not do this at all on the steel and much less on any other metals (duraluminum or- being an older Bayan potentially copper/brass- for the plates)
Be careful - Evapo-Rust does damage healthy steel if you leave the reeds in too long. I would use the brass brush first, to allow the Evapo-Rust to do only the finishing touch of removing the rust from any tiny pits on the steel's surface. An ultrasonic cleaner is helpful for this.

Don't reverse leather valves. They are meant only to work one way - with the flesh side in, and the grain out.

I am also leery of using guitar strings as boosters. They are certainly stiff enough, but they are not flexible enough. Proper boosters flex easily. If they do not, they change the pitch of the reed, and muffle it. I'd suggest buying the real thing.
My apologies for not reading the original poster's intentions fully.

New valves with proper boosters are a really minor expense given what you're doing and you're just begging for trouble with re-use wrong side out and improvised boosters suited for needle springs rather than accordion valve control.

Bossetto on eBay is a ready source of new boosters and valves and I'm sure there are many other supply houses out there.

There's "bravely going where no ...." and then there's sticking your bare hand in a hornet's nest!

Your hand, and I surely wish you good fortune in this endeavour.
Awesome project, though I feel a bit bad for the original accordion, I'd have used a less extraordinary accordion than this for your experiment.
Good luck!
There might be an issue with modifying the size of the air mass in the bellows area. The larger block of air might cause hesitation in sounding the lowest pitched reeds. And thanks for the book on Stoicism. I was reading it aloud upside down to better absorb the teachings.
I plan to use a steel brush to clean up the remaining rust. Does anybody see a problem with that ?
It seems like the standard choice would be a brass wire brush. You usually take a material for cleaning purposes that is softer than the base material you are trying to clean in order to minimize material damage.
Not only reed damage. but if you don't avoid scoring the reed plate, the performance of the reed is degraded, said Aldo to me once. I have used a bamboo skewer in a rotary tool dipped in oil and fine abrasive to polish away small rust spots. Don't forget the edge of the badly rusted reed will have rust affecting response. I would have a shim under the reed a raise it up slightly, then scrape along the edge with an industrial razor blade. A flat bladed jeweler's screwdrivers. with various width blades makes good scrapers for rust on the blued side raised with a reed lifter. For the top side. I mght use wood alcohol, hold a shim under it. and scrape flat toward the tip. The alcohol also deals with tobacco residue. Do not use a box cutter blade for scraping as that steel is too much harder than the reed steel. 3-in1 oil is a good rust loosener. WD-40 is great for loosening rust but it should probably be rinsed off afterwards. Oil should be wiped off thoroughly.