I never use metal outside corner bead anymore since I started using vinyl corner bead. Metal outside corner bead is garbage in my books now.
FIRST, nailing it on is a hassle, and you tell me how I'm supposed to remove those nails if I need to replace that cornerbead in future. You hammer the nail in so that you can't get anything under the head, and then you have to figure out how to pull that nail. Good plan!
SECOND, metal corner bead will bend and dent. And, once it's bent out of shape, it's near impossible to bend back into shape. The only option is to replace it, and that involves pulling the stupid nails again.
THIRD, you can't use metal cornerbead everywhere. On window returns, for example, condensation on the windows can get the drywall or plaster wet, and then the metal corner bead rusts and discolours the plaster and paint.
THE BEST thing to use for outside corners is VINYL corner bead. If you hit vinyl corner bead hard enough to knock it loose, then you will have hit the wall hard enough to have bent a metal corner bead. BUT, vinyl corner bead snaps right back into shape and you can just lift it away from the wall a bit to get some sticky mud under it and it'll stick back into place. Then just mud over it, sand, prime and paint, and your corner is fixed. Ya canna do tha wath met'l.
Phone around to the drywall wholesalers in your area and ask if they sell Trim-Tex. If so, THIS is what you want to buy:
http://www.trim-tex.com/catalog/cbeads.htm
Trim-Tex sells an adhesive spray to stick the corner beads on, but I find it better to "glue" them on with joint compound mixed with white wood glue.
All you do is mix up some white wood glue with water, and use that solution to either mix your powder or thin your premix to make a sticky joint compound that's soft enough to ooze through holes. Apply that to the outside of your corner and/or the inside of the vinyl corner bead, and press the corner bead onto the corner with your drywall taping knife. The excess mud will come oozing out the holes in the corner bead flanges. Scrape off the excess mud, and then either:
a) wipe down with a damp sponge to remove most of the rest of the sticky mud, or
b) apply normal joint compound mud over the corner bead right then and there, and allow the whole business to dry.
I prefer to just let the mud holding the corner bead on dry, and mud over the corner bead with a normal mud the next day.
And, if you ever want to remove that corner bead, just get a putty knife under one flange and pry outwards, and with a little effort, you can pry it off the corner cleanly.
I have 66 windows in my building and vinyl outside corner bead around each and every one. In the past 20 years, I have never had a vinyl corner bead come loose or come off or even create a problem for me, and I can assure you my tenants are not as careful with my property as a homeowner would be with his own property.
In my view, vinyl corner bead is the best kind of corner bead to have in your corners, it's the only way to go and I wouldn't use anything else in my own property.