We took our Bigfoot
pleated curtains to three dry cleaners and due to the backing, none would clean them.
My wife machine washed them on a gentle cycle with cool water and they came out clean. She then put them outdoors on the clothes line to dry. When dry she formed the pleats by hand and carefully ironed them on the untreated side. We can not remember for certain but we think she used "Zero" washing soap.
This was all trial and error, but the final result was all we could wish for. Even a watermark that was several years old washed out.
Hope this helps.