I agree with js55. It is important to understand the service environment to select the welding process best suited for your conditions. If the service environment will permit a flux shielded welding process that will leave slag on the pipe ID, either SMAW or FCAW are possible choices. However, if the operating system includes pumps or valves or other equipment that can be damaged by detached slag, then either GTAW or GMAW are better choices.
The optimum root surface condition will be best achieved with GTAW using a purge on the pipe ID. Consider using a U-groove with a zero root opening, 1/32 inch root face, 1/4 bevel radius, and 30 degree groove angle. As for the purge, dams consisting of rubber bladders or water soluable paper will minimize the volume that has to be purged and the time required to attain a proper purge. The purge time should be sufficient to displace the volume (of the pipe being purged) 5 to 6 times before the weld is initiated. Once the initial purge is completed, the flow of the purge gas can be reduced considerably to maintain an inert environment on the ID.
Best regards - Al