Yes, that elusive these days; but highest quality traditional method of finishing panels. Here, we undertake lots of panel repairs which entail cutting out sections of body panels, and the replacement of these with new steel.
While these new steel panel sections are invariably butt welded in position so that when ground flush, the welds are all but invisible, it is still the case that there will be imperfections in the shape and finish, around the joints, with some undulation and low spots which need truing up. Most these days would use fillers to even out the surface, but here, I think it best practice to lead load all welded joints and skim these off to form a perfect surface which is close enough to final profile to prime straight on. Lead of course is also the ultimate corrosion protector, meaning that repaired sections will be long lived!
More details of the process can be found in detail on the Lotus page 3 where it is was used to blend in new arch rings, and further where it was used for wing and door skin repair sections.