I do customs as a part time hobby and removing the paint it too much effort.  You can always prime over it with white.  I use ACE primer, because it dries quickly compared to others and when its dry it is very smooth to the touch.  perfect for painting/pasteling/airbrushing etc....
But make sure you start on a model that isn't so expensive so if you mess up, it's not a big deal. 

P.S.  when spraying the primer, go from right to left or left to right in short strokes.
try not to clump it on there, and if it does and you want the primer off.  Go to the closeset wal-mart and purchase a paint thinner. It'll come right off, but it will effect the model's paint under it.

I use primer to make them "all white". :) I learned from a youtube video as well, lol.

I use Krylon Primer, just because I also use the Krylon Matte Finish. On ebay if you search for "Breyer bodies" there are some already primed/unpainted horses available. :)