Thank you for your kind comments Zukipilot, BRD HNTR and TNTracker. Doing bodywork with lead isn't rocket science BRD HNTR. It's a myth that it is difficult. Anyone can learn. I learned that in just some days by my father.
Filling with thick layers of polyester is actually a problem in the long run. Plastic and metal, as steel, do have very diffrent thermic properties. That means that they expand and shrinks very differently when it turns from warm to cold to warm (in Sweden we can have +40 centigrades in summer and -40 centigrades in winter). So it's a big risk that the plastic will loosen from the sheet metal. Also if there is microscopic holes in the welding water will come up between polyester and steel by the capillary effect. So it's a big risk that the rust is comming back in a short time. Tin/lead, on the other hand, has thermic properties near steel and will attach by other physical laws.