Here's your video:
And your Album:
https://www.bikeforums.net/g/user/493077
Post some close-up photos of the vent holes in the stays.
I'm seeing a little rust around the holes, but everywhere else it appears light. Do you see any worms under the paint? Other signs of deep rust?
I'm thinking that some rust stabilization treatment might also trap the loose particles. Then disclose what you've done.