Unfortunately, if the tube has been down to air long enough for the gun to start rusting, it may already be a goner. Back when CRT rebuilding was a viable option, rebuilders would generally not accept duds that had lost vacuum, as they weren't considered rebuildable.
The aluminum coating over the phosphors is likely corroded as well as the shadow mask.
As soon as the air came in, the oxide cathode coatings started absorbing CO2 from the air to form inactive carbonates.
At the very least, the gun will need to be replaced. But if the mask or screen is damaged, the tube is worthless.
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