Manufacturers recommend keyholing but the coated stuff can be welded fine by consistent feed without keyholing. Widen the gap like suggested. You have to allow enough room for the flux to flow easily onto the ID or your wasting your time on SS. If the root isn't covered it will never hold up under corrosive service.
This stuff takes some skill. A few days of practice for skilled welder to get it right.
The other thing is, depending upon skill level run a double hot pass concentrating the heat on the sidewall. If you don't you will oxidize the root and you've wasted your effort again. If you feather edge instead of the recommended landing you will find your root is heavier and you will less likely oxidize the root. This may also allow a skilled welder to run a single hot pass but I wouldn't recommend it until you've done this awhile and have a feel for the results.
This stuff actually holds up well under ASTM A262 Practice E (copper-copper sulfate-16% sulfuric -Straus Test) if done right. Which suprised me.
I wouldn't think of this stuff as a replacement for purging though. Use it only when purging is impossible or at least extremely inconvenient and engineering has approved it for the intended service.