A laser welder works great. You use a wide beam at low power to just barely liquify the surfaces, that gets rid of the grow lines leaving a smoother surface in hard to reach areas. The areas you can reach are just file, emery, abrasive wheel as you know. Then a magnetic tumbler will get a fairly uniform overall semi-finish. Then stiff or soft brushes with your preferred polishing compound on flex shaft as needed in those cracks and crevices or areas to be covered by stones. Then final polish after setting.