I looked up scarfing powder on ESAB's web site. It appears to 100% iron powder.
If I understand what you've said correctly, you want to use the iron powder in the root of grooves with excessive root openings. You haven't stated what welding process is being used, if you did, I must have missed it. Base on the fact that you reference AWS D1.1, table 4.5, it appears that the welding process is SAW since it is the only welding process that lists supplemental powder as an essential variable.
I have concerns that the final diluted weld chemistry will match the target chemistry on a consistent basis. The chemistry will vary as a function of the root opening, the amount of powdered iron that dilutes/mixes with the electrode and base metal that is melted and mixed in the common weld pool. That being the case, how do you control the deposited chemistry of the weld and the resulting mechanical properties? A root opening that is 3/8 inch wide will result in a deposited weld chemistry that differs considerably from a root opening measuring 1/8 inch. As the chemistry changes, so does the mechanical properties. Ultimately, how do you possibly match the mechanical properties of the base metal on a consistent basis?
You have your work cut out for you. Going back to table 4.5; the reasons why items 8 and 9 are listed as essential variables relate specifically to the issues I have noted in the preceding paragraph.
Good luck.
Best regards - Al