Sid, sometimes a slight bit more pressure is needed from the backside to "persuade" the panel outwards, but I think I would continue to planish as you are doing to stretch the area as you go providing the area is remaining somewhat flat (not wavy) after your efforts. Then after completing this process along the full weld, address the lows. Another "controlled" stretching device that works well in this case would be a small shot bag, I have one made up about 5" diameter and pretty tightly filled. This wouldn't be appropriate for planishing a weld and HAZ, but for panel bumping afterward, tapping the panel from the inside with a spoon/slapper or hammer into the shot bag held on the outside will help to bring the panel outward where it needs to be. Use
light taps and work from one end to the other (as needed per panel lows) to do this in a slow, methodical, controlled method for better reading of the panel's reaction..
