Lead work is a dying art in the automotive industry. The prevalence and variety of plastic fillers has relegated lead work to an arcane practice performed by cranky old-timers in the back of dusty shops that have seen better days (but not us, our cranky old-timers work in well-lit modern facility). But lead has its place, especially when you need more than just filler. Lead provides structure, is much stronger than plastic, and lasts longer because it creates a superior bond to the base metal.
The first step to proper leading is surface preparation. Lead won't stick properly unless the surface is prepared by a process called "tinning". This is done by applying a thin layer of dry-tinning compound like Acro-Tin from Dutch Boy.
Before applying the tinning compound, the surface must be thoroughly cleaned. A scotch-brite pad, sand paper or roloc sanding disc will ensure that any surface rust and oil is removed. After sanding, dust off the surface and give it a final swabbing with paint thinner or acetone.
Before you start, be sure to wear gloves, eye protection and a respirator. Tinning compounds like Acro-Tin contain not only tin, but Zinc and Ammonium Chloride (a.k.a. acid). You don't want to get it on your skin, in your eyes or breathe the fumes released when it is heated.
To apply Acro-Tin the easiest way it use a small ball of steel wool held in a pair of long-nosed vice-grips.
Heat the surface to be tinned, but be sure not to get it too hot. A low even heat application is best. Then, warm up the tinning compound with the torch and dip the steel wool into it. The steel wool will pick up and hold compound until it is applied.
Use the steel wool to brush the compound on the surface while applying heat with the torch. As the heat is applied to the steel wool, the compound will melt and run onto the surface being treated. Continue to apply and spread the tinning compound over the surface to be leaded until you achieve a uniform coverage that looks like shiny tinfoil. Coverage should extend beyond the area to be leaded, but not too far as you will have to remove the excess later. Now you are ready to apply the lead.
One quick note before we move on. In these photos we are using a propane torch. This is fine for working on small areas like we have here, but is not recommended for larger surfaces. An acetylene torch is better because you can get more uniform heating of a larger area. The propane torch will tend to create hot spots.
Once the tinning is finished you are ready to apply the lead. The lead we are using is 70/30 lead, meaning it is 70% lead, 30% tin.
Again, heat the surface to be applied and the lead. Scott applies the lead by pushing and twisting the lead stick into the sheet metal. He does this until he has enough lead built up to cover the area he wants to work.
To shape the lead he uses a wooden paddle (made of hickory or some other hardwood). Wax is used to keep the paddle from sticking to the lead as he shapes it. A wood paddle is used so it does not get too hot to hold during the lead application, but you have to be careful, do not let any charred wood particles from the paddle contaminate the wax. If this happens, it they be transferred to the lead. Even a microscopic particle left in the lead when you are finished will cause paint bubbles down the road. Don't let anything get into the wax!
Shaping the lead is now just a process of heating it, spreading it with the paddle. Be sure to keep the paddle well coated with wax during the process to keep the lead from sticking to it.
Before you go home for the day, be sure to remove the excess tinning compound. If you don't, you can forget about your paint job later because the acids in will start rusting the steel at its edges.
We recommend a good metal cleaner like PPG's DX579. Apply the cleaner and hit it with a scotch brite pad to remove the excess tinning. Remove the cleaner with a clean wet rag. Scott prefers starting with a very wet clean rag to begin with, and finishing with a second rag that is moderately damp. Then dry it.
At this point it is a good idea to apply a self-etching primer to your work, making sure you cover the edges where the lead work give way to sheet metal. Un-treated, rust may begin there regardless of how thoroughly you removed the excess tinning.