Rust on Audi 80 Avant trunk lid - rear wiper leaking

Hi,
The tailgate on the Avant model shows corrosion below the rear wiper shaft. Initially, there were bubbles on the outside, just above the gap cover. After partially removing the gap cover, corrosion was also found underneath, with deep markings. Inside, in the area of the wiper motor, it initially appeared dry, but after removing the wiper motor, corrosion was also visible on the windshield flange, likely having migrated underneath the adhesive of the windshield. The dense layer can be lifted here with a needle.
The valve cover and engine block are dry, with only slight, localized discoloration on the flange of the engine block.
The construction of the shaft seal is similar to that shown in the technical article "rotating wash nozzle" by Bertil. It consists of a rubber grommet between the shaft and the rear window, with a sealing ring underneath (unfortunately not listed as a part in the parts list), followed by a plastic disc. According to the RLF (presumably a technical manual), the pipe can be pulled out after the nozzle and connector are removed.
The washing nozzle isn't rotating. I suspect that the seal on the shaft has likely deteriorated over the years. That's why I haven't removed the nozzle and the pipe yet.
If the rust can be removed while the window is installed, I have done so and applied a temporary coat of paint. The actual repair will be done in warm weather, likely involving a new valve.

Has anyone experienced similar damage?

hg
Herbert.