Ed, not that easy to explain in words because positional welding on a C&G course can last a whole term.
Both techniques are acceptable in normal use. The Yanks had a tendency to use what they called the stove-pipe technique, which is welding downwards - the snag there is having to keep ahead of your slag pool, so the runs are completed very quickly with a higher power setting than the (British) alternative. Trouble is, "penetration" isn't brilliant with the stove-pipe method. As one of my instructors used to say: "It's alright for welding stove pipes, but f**k all else." He didn't smile when he said it.
The "proper" way is the 'conventional' welding method, which involves welding upwards using what is still referred to as the Christmas tree method, which describes the shape you form with the rod (and that applies to MMA, Mig and Tig) in order to get full penetration on both pieces being welded, the trick then is to get a smooth action going and adjust your speed and wrist action to adapt to how the weld is taking, a good weld using that technique will see the slag lifting behind you and a smooth transition between the weld and the parent metal.
For insurance purposes (high/medium pressure work and structural components), when a test piece would be cut and one half hammered over the weld to bend the piece over on itself and the other half x-rayed, you'd be bloody lucky to get it certified using stove pipe.
But for this instance, stove pipe will do, but you will get a lot of either burn-through or being overtaken by the slag pool (if arc) until you've got the hang of things.
Incidentally, when I did C&G I was already Lloyds Class 1 and ASME 9, which is why I was trying (with the instructor's support) to get exemption from the first year, with no luck thanks to the muppet of a college principal. I spent most of the 1st year teaching the class instructor how to do conventional welding because he'd only ever used the stove-pipe method. I asked why the college was still doing that, given the insurance situation, and he said they didn't expect their students to be doing pressure or structural work once they'd qualified, which made me wonder how many of their alumni had been doing such work for customers who, in ignorance, didn't realise the structural implications.
Edited by joiner, 18 April 2016 - 09:26 AM.