Bill,
the lead slingers used different mixes,
tin = expensive =bonds to the base metal but brittle can crack
lead = cheap= poor bonding to the base metal stay flexible
The part I found interesting was the blue flashes when I was redoing a old fender. Looked like the flash in the mercury switch in a HVAC thermostat. Yep mercury was used in the "good" tinning butter! If you have time check out, damn they both pasted away this year, two legends in car customs. George Barris and Bill "the lead slinger" Hines on youtube.
Man this year has seen so many good people pass on. So many that I wish would go away are still dragging down the human race.
Back to the lead, you will see some guys lay down a good base of "higher tin content" then lay it on thick with the higher lead content save a few nickles and dimes on each job. I learn to lead in HS, then bought my own stuff a couple of years later when I could afford it, then the big lead scare came along. I still do some jobs but it takes me a while to get it to stick. save the vertical seams for last! There are many better than I will ever be, lots of room for improvement. I started a leading page on facebook it probably has 300-500 members but it's not real active.
Still have a bunch of soldering irons tinners and electronics type, even rebuild a Johnson soldering iron furnace from time to time. Kept one for myself.