Quote Originally Posted by Sean Hughto View Post
I have only the coarse/extra coarse Duo sharp for prelimianry shaping and back flattening. I've never even checked its flatness in any way, though unless it was grossly out of flat, I think it would still do okay on the coarse work I ask of it to get the steel ready for the finer stones.
If anything, I've found that having a flat reference on the coarser grits is more important than having a flat reference on the finer grits. Using coarser sharpening media of any type is going to remove more metal, and if it's not flat, that will leave a non-flat surface on your tool.

If I try to correct the non-flatness at the next sharpening step, by moving to a finer sharpening medium I'm slowing down my metal removal rate, which means that it takes longer to correct the non-flatness than it did to cause the non-flatness in the first place.

By making sure that you have a flat reference at the first steps of sharpening, you'll speed up the sharpening process overall by a lot. I've found that ideally, the majority of sharpening happens on the lowest grit that I'm using, and the following honing stages go by pretty quickly.