I cannot speak for the white panels, but I restored my black door panels with SEM dye, also.
I have heard good reviews of the SEM prep products, but I just scrubbed mine good with dish soap and water (First with a sponge or rag, and then with a Scotch-brite pad). I rinsed them well, let them sit in the sun for a while, and then applied 2-3 light coats of SEM dye (paint).
I used the same technique for my windshield post covers and plastic lower dash corners, too. They look like brand new - So I would expect the white to work the same. Best of luck.