It’s going to be slow and frustrating to get down to bar iron no matter what you do. If the funds permit, I think I would remove the fence and send it off to a commercial sandblaster. They will clean it up and you could have it repainted in a powder coat, it will last for decades.
Cleaning in place, I would suggest a needle descaler.
They come in both electric and pneumatic versions. If you have a good compressor with sufficient volume I would suggest pneumatic. If not, electric will work just a good.
Ron