Lewis, as I stated on the other thread,
I recommended more Electro.
It did not appear that all the imbedded rust had been effectively removed,,,, yet.
I always assume that dark spots are still harboring 'bad' rust that still needs treatment.
You may actually find pitting underneath those dark spots, but at least you will know you've gotten it all.
Best thing to help avoid flash rusting is to do final wash downs with COLD water. Thoroughly dry the piece with paper towels, until they come away clean,,,, then into oven for drying at 225°. (if using a gas oven, always vent the moisture from the oven by cracking the door open for a minute or two, before placing iron inside).