Its been ages since I leaded, but used to do a fair amount back at the resto shop.. Honestly, some Dynaglass and a skim coat of bondo is nearly as durable if done correctly imo. At least I have no fear of it coming back..
But here's some thoughts on what I remember..
First off, I haven't used the leadless that Gauder mentioned, but have heard limited reviews of it that it works well.. Probably has some different tendencies but overall pretty much the same..
First thing you need to do is thoroughly grind the whole area then tin the area to be filled with a tinning slurry.. Comes in various forms, I'm mostly familiar with the powders.. You mix it with water to a consistency of say, somewhere between chocolate syrup and ketchup... Coat the area to be filled and go an inch or 2 farther out for assurance.. A stiff bristled acid brush works well for this..
You then take a torch, I generally use a large tipped brazing setup, and heat the area. Keeping the torch a couple inches off the panel.. Basically your getting the solder to melt and stick to the panel. It gets shiny when its melted and done.. Wipe a wet/damp rag over it to get the flux off after its tinned, don't wait too long, should still be warm..
Then you want to start applying lead.. Warm the area your going to apply it to with the torch, then heat the tip of the lead and mush it on.. Apply heat to the rod end as needed, and a twisting of the rod works well for getting it come off.
Once you think you have enough lead, you then take a wooden paddle to it to smooth it.. You can just heat the end of the paddle to help it not stick to the lead (just a little, it IS wood).. Some guys have been known to use beeswax on the paddle to help it smooth the lead out without sticking, but you can just heat the wood some and that does it too..
You apply heat to areas of the lead and drag the paddle over it to smooth it.. It helps if your finishing your "drag" on the edge of the repair, that way you "cut off" the lead. Don't get the lead too hot or it'll just roll off.
Proper finish is done with a lead file. Most physically demanding, but for certain areas the cross hatching pattern of handfiling will insure good bodywork.. But the lead can be shaped with various tools from grinders to DA sanders.
It's also a very good idea to metal condition the lead after to insure proper primer adhesion... I like to use a couple pisscoats of zinc before regular primer..
One of these days i'll find a reason to do some again.. It was always quite fun... Just the speed of fiberglass and mud takes over in a production facility.. Tho if set up and doing it daily, lead is nearly as quick. Just more difficult and taxing when your doing all that filing..
