Making yellowed WS white again?

I've just acquired a WS DDH-01 head and I'm guessing it's pretty old as it's quite yellowed compare to my WS girl Akari.

 

I had thought about sending this off for some modding + a faceup, but before I do my big question is: Is there any way to "restore" yellowed WS vinyl?

Have you tried cleaning it? Last year I thought my oldest WS girl was yellowing but it turned out it was mostly just dirty. After cleaning she looked nearly new again.