tell us your grit progression, and i assume you are using new sandpaper ? if you still have 150 grit scratches in there, it could show up as whitish areas, next time try the 45 degree alternate sanding directions everytime you change grit, like you hand a sand a blade, to make sure all the previous scratches are gone. then there is always the possibility that you are sanding correctly, and the white is just in the material all the way through the tube.. in that case i would wipe it really well with acetone, and color the white with a black sharpie.