I use a 1/4" die grinder, with a stone burr on it.
I smooth the seam out from both ends and finish it off with a half round file by hand, where I can't reach in the middle. Then weld it on.
Sometimes the galvanizing is rough where it has pooled inside the pipe (when it was made), or where the top has been flared out and needs some attention as well.
A few laps of fiberglass filament tape around the mast and about 75mm cut from a motorcycle tube to keep the crap out.
Hope this helps,

Chook