Moisture can not get under lead like it can bondo. I watched them lead every fifth Avenue in the late eighties that came down the line. I made the tools out of maple to apply it. It was a true art and yes not always perfect.
Fred the GTX had a bad case of bubbling at the lead line when the prior owner painted him; for some reason, there
was a reaction between some part of the paint process and the lead itself that happened nowhere else on the car -
and only on the drivers' side.
When I had him into the body shop after my little escapade with the garage door frame in 2019 (I have a thread on
here somewhere about the whole painful process) and they were fixing the dent in the quarter for me, they offered
to fix the lead line bubbling as well and I took them up on the offer.
We wound up fixing not only that mess, but a couple beginning rust spots at the seams of the rockers under the
doors on both sides too. Nip it in the bud before it became something later on.
I still have the molten blob of lead from both sides of the roof that they took out and replaced with filler. It was
pretty surprisingly a large amount from both sides all told. I was there the day they did all that, too. Fascinating
Paint match was perfect, once they gave up on their water-based nonsense and went to the supplier to get the
exact same brand and formula mixed that I gave them (since the PO gave it to me) - and no more lead mess: