I have hand-washed whole balls of yarn, even a skein or two....gently squeezing and sloshing through the water. for soap I usually use Eucalan, but if really tough dirt spot like your rust, you may need something stronger, like soap. Then I rinse, wrap in a thick towel, step on this to get most of the water out. I let the entire ball or skein dry in the sun, turning it every hour or two.
Good luck with yours! I hope you can save it.