Some of the links on this forum allow SMF, at no cost to you, to earn a small commission when you click through and make a purchase. Let me know if you have any questions about this.
That is called mill scale, it is a pain and part of the rolling process....The best way to get it off of a large flat area is sandblasting and use an angle grinder to bevel the edges or grind where you are going to be welding.
I hope others weigh in too, but I have heard of most guys doing a good burnout of the cook chamber and then season with cooking oil afterwards. I don't think most people sandblast inside just to get the scale off.