Just finished a remodeling job where I took out an existing closet doorway and made a large opening with an arched top. Straight sides with a circular arc on top. I used "bullnose" corner bead to make a rounded edge all the way around.
The sides were easy: I used the metal/paper bead, which has two pieces of paper (same as drywall tape) bonded to a thin galvanized curve. I first tried to use this material for the arch, cutting segments into the bullnose with tinsnips and then bending the bead to the arch. I *almost* managed to make this work, until I noticed I'd nailed it up wrong so that there was no way to hide the nails in the final result. So I ended up tearing it out and replacing it with the flexible plastic stuff.
Using that stuff was a challenge, too: while it went up nicely, when I applied a coat of mud over it, it left telltale "railroad tracks" where the slices were, which looked pretty near impossible to get rid of. So I ended up mudding the bead and laying a couple of layers of tape over it, which I first soaked to make more flexible. This ended up working OK, though I had to work over the tape pretty well to get out the wrinkles, then cover it with another layer of mud to cover the wrinkles I couldn't get rid of.
There's got to be an easier way. Anyone know how to make such a rounded-edge arch?