door panel rehab

[67-72 Sub K20]

Hi there,

Looking for help with door panel rehab. All four original panels for my '72 burb are in good shape except for the fact that that the PO sprayed some type of black paint on them, probably many years ago. Original color was white/Parchment. Much of the black is coming of with air, washing, and a little acetone. The rear panels are looking almost brand new. The front both have some yellowing areas. Not sure what this is. Cannot seem to get the yellow off and the more I wash/ scrub it seems that the color in the grooves of the texture is being removed. Does any one have experience cleaning these panels?
I could paint them properly but would rather get the black paint off and keep original if possible.
Thanks for any help!
-Mike