Welded keggle question

I just got my keggle back from the welder today. I am planning on giving it a good wash in BKF. Should I let the BKF rest on it for any amount of time or just scrub and rinse?

Aside from that is there anything else I should do? There are some flakes from the top being cut out that are stuck to the rim, outside the boil area, (picture follows) should I worry about getting rid of them due to rust issues?

Pictures of the welds and such:

I might take a piece of sandpaper to that leftover residue. If it doesn't come off I wouldn't worry too much. As far as cleaning, Just scrub and rinse well and you're ready to go.

I used my Dremel to get rid of the build up and then scrubbed the keg with Bar Keepers Friend. The keg is much more clean now, but when I was wiping the inside down with a paper towel the towel had a grayish color to it when I was done. Does this mean I didn't sufficiently wash the keg?