When welding mild steel, what is the most likely cause of dull welds?
Running several beads, I will have one that looks nice and shiny with the
hint of different colors -- almost iridescent looking. The very next one
will be flat gray. Not visibly porous, but not healthy looking.

This is while welding in subframe connectors on an older vehicle. Are the
dull welds structurally inferior? Should they be ground out and redone?

Thanks!