I don't know much about this product but its really frustrating when you entire show stable is white except one door. I tried all sorts of things to whiten my door so...since none of those worked I may try to find this product. Thanks for posting about it.
The doors are small enough that this method might work for you too:
http://www.mlppreservationproject.com/misc.html#AccessoriesScroll down a bit to the "yellowing" sections. Don't soak parts with stickers.
Hope that helps!