More info: Everything I am welding is mild steel. I am building a Dana 60 for a car which involves cutting the ends off a Dana 60 housing and welding car ends onto it. The car ends I am welding on are about 1.5" long so the rest of the tube is stock Dana 60. I don't know for sure what material the tube is but they are mild steel (thick wall tubing I think) and commonly MIG welded or arc welded with plain welding rods (6011's, 7014's, etc.).
I am not touching the cast iron center section so I'm not welding on anything out of the ordinary. I believe all rear end housings have the ends welded on from the factory.
Thanks,
Clay
My advice is to use E6010/11 for the first (root) pass because it has good penetration, and E6012/13 for the subsequent ones, or at least the last (cover) pass.
The advantage of E6012/13 over the 6010/11, apart from the one you mention (the slag is easy to remove), is that they have a good looking finishing, contrary to E6010/11, whose finishing isn't that good looking.
This is an important feature when the welding will be seen by the people, as in your case.
Giovanni S. Crisi
Sao Paulo - Brazil