Having just come from a class on forge welding, I'll weigh in with my newly acquired experience.
After getting the scarfed pieces up to temp, we removed them, wire brushed the area and fluxed them quite thoroughly. for the flat stock scarfs we also used a welding compound called ez flux, that has metal shavings and other interesting but unknown items in it. Darryl called it "GLUE" you put it on after the regular flux then when you put the two pieces together before your first strike, they DO stick together, briefly, allowing you to get that first tap to set the weld. Centaur forge carries the EZ Weld compound. I'll be ordering some soon, though I don't have any major welding projects lined up.