Our grandfather worked in a foundry doing plating and casting in metal. What job title includes these skill?
I would suggest a metallurgist. Alternatively, steelmaker, moulder, welder, tinsmith.
From my understanding metallurgist involves both plating and casting, as well some elements of engineering. Other names are more specialized, and I included them into the roster hoping that one of them can ring a bell and somehow help the OP to decide which one fits better. I also must point out that I'm not a native English speaker.
He could also be called a Foundryman.
A person who plates metals is called a plater, and if he uses the processes of electrolysis to do so he may be called an electroplater.