Hi Steve!
Many times people will suggest that individuals start out their forge welding with cable because "it's easier" than regular laminated materials.....NOT SO! I actually think that cable is one of the more difficult materials to forge weld. If you think about it, you have a whole bunch of tiny round rods, wrapped loosely around each other, and they just try to keep sliding past each other when being forge welded.
Those small pits you mentioned are indeed weld flaws...and they will cause not only rust issues, but also integrity issues.
There was a time when I made a great deal of cable damascus, but as I became more experienced and gained better understanding, I realize that I was handicapping myself by using cable, and not giving my customers the best I could do.....I almost never forge weld cable any more. I personally think that unless a person has a good supply of NEW cable, of known element content, that they are just spinning their wheels.