All classic vehicle restorations will need to undertake welding and sheet metal fabrication. However to get to that perfect presentation ahead of painting then you will need to use a filler. The ideal option for this application is Body Solder, which doesn’t absorb water and subsequently break down, covering a multitude of sins with its permanence and flexibility.
Car manufacturers began using plastic filler during repair due to health and safety concerns, however more recently there has been a revival of lead leading due to its greater durability, malleable nature and suitability for deeper filling.
The technique of Lead Loading is fairly simply using solder paste to tin the steel to encourage adherence, cleaning and then loading the lead into the area using Grade D Body Solder and a Blow Torch. It does however take practise to master the skill, particularly with the correct level of heat to apply.