never have had a lot of trouble with the old lead, but I usually grind a bit of it off and start with the Evercoat filler with the fine strand 'glass... Evercoat also makes a product called Metal to Metal that is both air and moisture proof that works good to seal up leaded joints, just a light coat of it though!!!!

Two schools on applying filler, some say use it over bare metal, others say use Epoxy primer first.... I've done it both ways, usually do the Epoxy primer first after sandblasting and lots of cleaning time to seal up the surface of the metal and prevent surface rust from forming....