As far as I know on the AD vintage, there were no stress areas with lead that required any flexing beyond what a premium body filler can do. They used lead because that was the bondo of their day.
Rage Gold or Rage Extreme altho expensive completely resolve the issue. Remember, you must do proper metal work prior to filler. Todays bondo is so much better than in the olden days. If you do it right, you wont have much filler in any case. The lows need to be worked with metal to keep the depth of the filler very shallow.
What most professional resto shops do is put a 1/8" coat of premium filler on the entire vehicle. Then with long boards and lots of hand work, sand almost all of it off. Most of the auto body technical schools suggest this method as well. If you have more than 1/8" of depth, really you need to redo the metal work.
I use long boards as long as 36 inches to really get the panels smooth over the long areas. I have never used air tools or any kind of methods other than hand work. I don't plan on going that way ever. I 'feel' my way through the process and the end result is very nice. Not that the professionals that use power equipment aren't better. They can be.
Bottom line is, they didn't have todays superior fillers in the lead days. Not to take away from the guys who believe in lead.. they do amazing work too.
