Uphill and Downhill Welding

Aside from the obvious, whether you start at the bottom or top of the seam, direction matters! Always start at the bottom so that your overlap/puddle lays on top of the bead and fuses with it properly. If you weld downhill the puddle flows into the cold seam and doesn't fuse with the surrounding metal resulting in a weak or non-existent bond.

Consider the effect gravity has on the liquid puddle of metal as you are welding. It wants to flow "down"hill.

Now read the above post again.

down hill welding is never appropiate for any type of welding. the slag and impurities will run back down into the puddle resulting in a weld that has slag and impurities embedded in the bead.

I have seen downhill welding advised for very thin metals. Maybe at the low heat settings the flux doesn't run around and down into the puddle. It sure is difficult to weld thin metal uphill!

For jobs where strength was not as important as looks and speed, I have successfully welded downhill with certain 'jet' rods. The technique was to keep the rod deep into the weld, to keep the flux out, and angle the rod up into the weld. Jet rod likes to be close to the puddle, and leaves a pretty weld.