When going through the restoration process, all aged undercoating should be removed. Undercoating is the factory protectant that is sprayed on the undercarriage. Over the years, this protectant dries out and cracks, allowing rust and corrosion to hide underneath. It is not always easy just to scrape it off. If the coating does not easily scrape off, try using a propane blow torch and heat the undercoating until it turns black with red-hot spots and smokes. This allows the undercoating to soften up. Then, it should scrape off easily. Once the undercoating is scraped off, use solvent and a wire brush to clean off the residue. With the undercarriage properly cleaned, you could then re-finish the undercarriage with red oxide primer. You could also use rubberized undercoating which is widely available in aerosol spray cans.