I don't disagree with anything mentioned by Steve, other than to add the following:

1) abrade both the ID and OD surfaces. Wire brushing is not aggressive enough. A carbide rotary file or abrasive wheel are best.
2) each weld bead should be abraded to bright metal before depositing the next weld pass. Again, wire brushing is not aggressive enough.
3) use stringer beads (no weaving at all) with high amperage, low voltage (tight arc). The weld puddle is "gooey", i.e. doesn't lend itself to weave beads.
4) pay particular attention to the toes of each weld pass. Blend in any cold edges that appear to have a rounded or deep crevices. The weld puddle does not penetrate, so it is important to prepare each layer for successive welding, no deep crevices!
5) maintain the purge until the root surface is below 300 degrees F. Check a previous thread on the subject of purging.
6) I prefer a tight root opening, i.e., zero, and a root face no more than 1/32 inch thick. This minimizes leakage of the purge and lessens the ability of air to enter the system.
7) weld the joints starting closest to the purge source and work toward the farthest end of the spool.
8) use vertical uphill progression if welding in position.
9) keep it clean. Wear clean gloves, clean the filler metal, clean the pipe ID, OD, and weld groove with acetone followed by isopropyl alcohol, don't touch anything with your bare hands after cleaning. This also means cleaning the wire brushes, rotary files, etc.

Best regards -Al