I am a self employed welder and have removed many broken studs and bolts over the past 15 years. I can tell you that I have had my best luck using good old E7018 or E11018 rod. I have tried the rods in the post and they make such a narrow cross section of build up area that it twists off too easily to remove all but loose fitting bolts. Any bolt or stud broken below the surface requires a great deal of skill and practice to remove without welding it to the wall of the hole. That being said, if you fail to remove the broken stud using the build-up rod, you'll have a bigger problem than what you started with.
On a related note, please-please don't be tempted to drill & use an easy-out, as it will leave you with even less material to work with. Believe me, I've had to fix that kind of mess more often than not. Bottom line, unless you're an experienced welder, you're best off bringing it to a pro.
