I have started patching up my frame, got the first patch(12gauge) in place, ground the weld smooth on the outside with my 4.5" grinder, but it won't reach the welds on the inside so I need to use my die grinder. I havn't used it much though I have a range of stones/wheels(?) and burs for it. What is the preferred grinding tip for a die grinder for grinding weld down? I have grey, pink, and white stones and various metal burs, various shapes if all. Thanks.