FYI, over several years time my turtle's shell went from having a little bit of white - like yours, Andrew7769 - to being almost entirely white today. The white seems to have damaged the outer layer of his shell. He's never shed. I've taken him to two vets and neither seemed to know what the problem was.
Until about 6 months ago Spot didn't have UVB light and he didn't bask much. Since I found this site I've made a lot of improvements and now he has plenty of good light and he basks a lot. But his shell still isn't right.
I saw a post from Reptilegrrl where she lowered the ph of her turtle's water and his shell got a lot healthier and he started shedding. Our water - like Reptilegrrl's - is very alkaline. I'm working to lower that ph (it's hard to do!) and am hoping that it will finally cause Spot to shed that damaged shell so he'll be all healthy again. See the post at
http://www.redearslider.com/forum/viewt ... highlight=
Some of Spots scuts were lifting before I started lowering the ph and I think a lot more of them are lifting now. I've broken off some small pieces and the shell is dark and healthy underneath.
Andrew7769, if your water is very alkaline you might think about Reptilegrrl's experience. Maybe lowering the ph will help your turtle.