I'm still one of the old geezers that used lead before Bondo became popular. I still do a little of it ever once in a while to show people how to do it. However, this is not a spot for a filler. Lead or Bondo. You need to cut out the bad stuff, weld in a piece to replace what you cut out. Grind down the weld, and then use either Bondo or lead to fill the imperfections. Leading and soldering is 2 different things. Soldering involves heating the parent metal until it melts the solder to flowing. Leading requires heating the parent metal and then applying lead to where it is not fully flowing, but spreadable like butter. I would patch it and then use Bondo to fill the imperfections.