I don't have experience sharpening that way, but as I understand it, convex sharpening is done (industrially) with a slack belt sander, rolling from near vertical. I'd guess that it tends to make the transition between the side to the beginning of the edge grind a bit inconsistent. Horn Dog has more experience, so could provide a more definitive answer. I bought a low-speed belt sander (+ leather strop belts and compound to fit it) specifically marketed for knife sharpening, but I never set it up -- it's still in the unopened box in my garage. The only fully-convexed knives I can recall seeing had polished blades; that doesn't mean there aren't any coated ones, but none come to mind.