[pulser]

Also vote for uphill.

Gravity pulls the puddle down. In downhill the puddle will tend to flow down in front of the arc unless it is keep cold enough to prevent this. When the puddle is infront of the arc, the puddle gets hotter and runs more. The arc is not melting the base metal so penetration is shot. Pulsing, as in orbital welding will allow downhill welding.

In uphill, the welding heat can be substantially higher because you are contiually moving away from the puddle as the puddle sags in the opposite direction, allowing the puddle to solidify and form a shelf helping to support the ongoing pool. The arc is melting the base metal ensuring good penetration and fusion.

 

[pulser]

Never say never, but it makes sense.

360 degree orbital tube welding is done without wire on stuff less that 1/8", but thicker material that requires a joint, wire feed, and multiple passes is done "double-up", start at the bottom and go to the top, then go back to the bottom and do the other side.

If I recall correctly, I think the root pass on pipe is sometimes done downhill with stick (E6010).

I've seen machine cladding of SS downhill with pulsed MIG, but it had to travel around 60 ipm, while oscillating side to side, to stay ahead of the puddle.