Phil V wrote:
Part of the process of using lead as filler is "tinning" the metal with an acid to prepare the metal to accept the lead. When the bodyman applying the lead is done filling the spot then that tinning acid has to be washed away so that the metal is clean and acid free. What happens occaisionally is all the acid isn't washed away and it WILL cause problems with rust and adhesion.
Some milder acids such as phosphoric acid can just be rinsed away, so just to be clear, tinning acid is much stronger and has to be neutralized. To do that the acid is washed with a solution of at least 1% baking soda to neutralize it before rinsing it off with clean water. And that should be done very soon, because even the fumes from the tinning acid will rust the surrounding metal over a few days. And just before painting, it is also wise to use lacquer thinner with a hand held wire brush over the lead followed by G&W remover , to be sure any sand or other contaminates are out of the soft lead.
IMO, this is the reason for problems when painting over lead that has never had a problem before in all those years. Lead is like most other products, it needs to be understood before using it, or painting over it. And this is why I don't recommend most people using it.