How can I get sheer (used to be White) sood dingycurtain white again?

Borax. Soak

Baking soda and vinegar work for me. See if this helps!

https://home.howstuffworks.com/home-improvement/household-hints-tips/cleaning-organizing/how-to-clean-synthetic-fabrics6.htm

https://www.hunker.com/13422521/how-to-get-polyester-sheer-panels-white

Hello! Why not use Rit Dye. It comes in white. It won’t make something cream colored white, but it can make your yellowed white curtains look brighter and better. I would suggest using a couple of packages of Rit color remover first followed by a couple of the Rit Dye packages in white. You should have a successful outcome! Good luck!

I would try liquid shout and boiling in a pot of water. Then wash as usual in machine.

Get some oxyclean and use it just like they do in the commercials, it whitens a lot of white things with a good soak and is safe for most fabrics. The label usually tells how much to use in how much water, you can use either the liquid or the powder.

Have you tried borax?