Using lead is not for everyone and there are a lot of things that can go wrong and make you wish you had never tried it. But there is a lot of bad information on the internet and on YouTube so it makes sense to offer a better way of doing it for those who insist on using it. The important thing when doing body work is to get the metal as straight as possible and just use filler to finish it off. The polyester type body fillers are so good today that lead should only be considered in certain situations in my opinion. If you do decide to try it, please take all precautions to protect your skin, eyes, and lungs, learn all you can about it, and take the time it takes to get it done right. Its a slow process but it can be a very rewarding experience.
This video was a big help to me in getting good results with body solder and well worth the $30. There is a preview of the video on YouTube but not much useful information on that preview, its just a teaser.Lost Art of Body Lead Work
One of the most important things in body solder is getting the metal clean before you even start. Sand blasting is not nearly good enough on rusty areas and even wire brushing after blasting may not get all the specks of rust out of the pits, and any little speck left on there can be a problem later because the lead won't stick to it. So if you are working on a panel that you know is clean and rust free, then sanding and a clean wire brush followed by a wax and grease cleaner will be good enough. But I like to use some type blasting abrasive on rusted areas and then use phosphoric acid with a small stainless steel brush by hand to agitate, and the rust pits will even require a metal pick to keep digging at the specks and then SS wire brush, while keeping the area wet with the phosphoric acid the whole time then rinse with distilled water.
Blasting is good enough for metal that will be coated with epoxy, but after you brush the phosphoric acid on you will be able to see the rust that blasting leaves behind, and for lead work it has to be completely clean or you may have paint failure.
When rinsing the phosphoric acid you want to make sure it is thoroughly wet with the acid and that the acid has been wet and active for more than 15 minutes before rinsing. For full panels or larger areas I like to scrub the wet acid on the panel with a red scotch pad to get rid of the acid etching (loose dissolved metal residue) and make it a cleaner finish, then make sure it is totally wet with acid before pouring the distilled water over it to thoroughly rinse. If you are working on just a small area you could use a hand held stainless steel brush to scrub the acid wet metal clean.
Eastwood and others sell tinning butter to use for tinning flux but I had some trouble with that and learned from Steve Frisbie on the video that Stay-Clean liquid flux made by Harris works a lot better, and is also the only flux that Wild Bill Hines recommends. Any welding supply can order this.Stay-Clean liquid flux
My problems with the tinning butter happened with my first attempts many years ago and may have been just from overheating the metal---which I was unaware of doing. So you will have to decide for yourself what you want to use.
When the stay clean liquid flux is applied to the warm metal and heated up it will show any areas that are not clean enough - see Steve's leading video - and in addition to fluxing, stay clean will also do some cleaning, unlike the tinning butter. It is possible though to overheat the stay clean or tinning butter burning the metal and the back side will be a shade of blue, at which point the lead will not stick and the metal will have to be re-cleaned and re-tinned.
Tinning butter will bubble and turn darker when it is ready to be wiped, but the way to tell when the metal with stay clean is warm enough is to touch the lead bar to the metal, when the metal is warm enough to melt the lead, then it is hot enough.
Stay clean is a hydrochloric acid which is a really nasty acid and not to be taken lightly. For instance, if you were working on the hood and left the acid on it over night, the fumes might go down under the hood and rust the bottom side. So its important to wear protection and to neutralize it as soon as possible and make sure you don't spill it because the fumes are almost as bad as the acid itself. You can find the way to neutralize it in the link below by clicking on to (resources) and then (spec)Neutralizing Acid
My process for neutralizing is to wait until the lead cools and before using the file I just use a solution of baking soda and warm water on it and dry, then phosphoric acid to be sure of neutralizing the flux using a small stainless steel wire brush by hand to get into all the nooks and crannies good, and then just rinse with distilled water, which is neutral on the ph scale so it will help prevent flash rusting.
Personally I don't like to use lead any where away from the edge of the panels or some other reinforced or high crown area because of heat related distortion. For instance if you are leading out in the panel somewhere away from the edge, then when you stop leading and after it cools down and you finish filing, you may notice that you didn't go quite far enough with the lead so you get the torch and go a little farther, but when you get through with that you may notice that you still didn't go quite far enough, but it is actually just the distortion from the heat and you will always be chasing your tail---IMO---it just depends on how much crown the panel has or how its reinforced and how thick the metal is.
You want to heat a broad area around the repair area to help keep distortion down, localized heat is much more likely to cause distortion. The paint and rust stripping company that I use locally has a very large oven that they use to burn the paint and undercoating off and to do that they heat the metal to 800 degrees F. The oven is programmed to raise the temperature very slowly to 800 degrees and then very slowly bring it back down, so its the speed at which the temperature is changed and the fact that the whole panel is heated that keeps it from harming the panel.
Another thing to watch out for is overheating the metal and/or the lead itself during the lead application or while shaping it with the paddle. If the lead turns liquid, its too hot, and that can happen without overheating the metal when the lead is built up thick. If you watch the video of Wild Bill working the lead, you can see that he doesn't spread the lead he pushes it and never gets it real soft. So this is why the metal and the full thickness of lead has to be heated to work it properly. Spreading the lead like icing on a cake will cause pin holes and bridge over some areas that will result in a low area after filing.
I was once leading the edge of a filler panel between the back window and the trunk, and kept working it for awhile and after filing and checking my progress with the trunk lid down I found that the gap was worse than before I started. That was because of heating a small panel too long and getting it too hot which caused the distortion, so sometimes you may have to let things cool down before you continue. In that particular case I had to replace the panel.
As for adding the lead to the metal, I like the way Wild Bill does it rather than the way Steve Frisbie does it, because I think it takes more heat with Steve's way. But either way if the lead crumbles then its not hot enough, you should be able to push the lead into the metal with a twisting motion and have it stay put without crumbling. For adding lead to the tinned panel, it doesn't have to be hot enough to melt, as in tinning, the metal just needs to be warm enough for the lead to stick. So after warming the metal, then its a balance of evenly keeping the metal warm and heating an inch or so of the lead bar to the point that the lead is ready to fall off the bar and then you push it into the metal and twist to break it off. Then keep adding lead like that until you have plenty to get the repair done.I also like the shorter paddle that Wild Bill uses, I just cut one down to that size
There is an alternative to use lead and that is lead free solder. It has a workable (or "plastic") range of 428° to 932°F (220° to 505°C), though the best working range is 535° to 660°F And the lead base body solder has a much lower working range of (361° to 489° F) which for me puts lead free at a strong disadvantage. But there are advantages to lead free as well, its much safer for you, and there is no worry of embedded sand. Lead free solder also offers 50% more strength, if the occasion ever arises that you need more strength than lead base solder offers, which is 6140 PSI tensile strength.
I wouldn't advise leading a seam unless the seam has been welded solid, because there is the possibility of trapping flux acid in the seam which could cause you problems down the road. Yes, its true the factory did it that way, but those guys were working with lead everyday and knew what they could get away with.
Eastwood has a 1/4 lb bar of 30/70 lead (30 percent tin 70 percent lead) but I like the 1/2 lb bar that TP tools carry. Your local welding supply can also order what ever you want. East wood does have a diffuser tip for your acetylene torch that will give you the perfect flame that doesn't require any oxygen, and will help a great deal in your efforts to keep from over heating the metal.Eastwood torch diffuser
It is best to use a file for shaping the lead because if sand from sand paper gets imbedded in the lead and you paint over it, that will probably cause a paint failure and could be one of the reasons a lot of guys have trouble painting over lead, especially after sand blasting. When filing the lead it should feather out on the edges very much like plastic body filler, but if the thin areas of lead start to curl up, then that means you don't have good adhesion and all the lead in that area will need to be removed and start over, it was probably over heated.
The file leaves a smooth surface, so I like to use carbide abrasive to put a profile on it, then after prepping all the metal for paint I clean the lead again with phosphoric acid and a hand held SS wire brush and rinse with distilled water. You can see what Kent White (The Tin Man) says about sanding lead here, post #5 All Metal Shaping Topic
It takes a lot of patience, practice, and commitment to get it right. But when its right, nothing compares to it, you can bend the metal or even work the leaded metal with a hammer and dolly without cracking it.
For clean up after filing it is advisable to use a vacuum cleaner instead of a broom to keep the lead dust out of the air you are breathing.
Protect yourself and be careful with the torch, don't start a fire, good luck!