Yeah, it's pretty standard to see a line on the second pass... if you leave the mill head locked and do another two passes the line should almost disappear. This is due to the cutting heads feed rate. Not all the material you want removed will happen in one pass.
If you had to make two passes on the surface to get it all. Make the same two again WITHOUT changing your mill head hight.
It will smooth up the surface and some of the swirling will disappear.
You should then be able to lap the rest of the milling marks out and get a mirror finish.
Just check what BladeRunner has done...
