My method would be to use Whink rust remover. This is really strong stuff (and toxic), and a little goes a long ways fast. I would dab the stain with a Q-tip dipped in the remover. The stain should fairly quickly come out, and then I would carefully but thoroughly rinse the area. The rust remover can eat through fabric if it is left in. When you are done, you are likely to have a bit of a water ring stain in acetate, but you should be able to remove the water ring at least adequately. I learned this method from people here who vouched for it, and it worked for me as well. Water rings can sometimes be lessened by rewetting the stained area with a damp cloth. Moisten the area enough to obscure the ring. Then use a dry cloth, a blow dryer, or an iron (on the setting appropriate for the fabric) on the edges of the dampened area first. I like using the cloth best. What you are doing is dissipating the edge of the ring, then quickly drying the area so that there is not a new ring.
If you have just rinsed the spot you used rust stain remover on you can skip letting it dry and finding a ring stain. Just use the water ring-removing process from the start.
I hope that helps!
