My theories are twofold. As I mentioned either added before or after the welding was done. I Would say probably after. If seen it enough times to know its not something post war done. They either applied this to seal the weld or something else all together ? Slag or whatever coated the rod or stick when welding ? As I said I welded a bit in high school and certain welds you use a pick hammer to de-slag the weld. Arc I think ? These is obviously looks to be electric welding even though a good gas welder could weld steel even better. Not sure 100% just figured id highlight this.