Dye Penetration Test is a quick way of finding surface weld defects. If the weld leaves even a micoscopic ----- on the surface you are sure to catch it with DPT.
As for the use of wrong electrodes, you must have failed in inforcing welding schedule for the perticular joint. Generally agencies dont care about the welding schedule. if the target is tight they will use large electrodes with higher current just to finish the job in time. but in my experience anything done in hurry is compromises the quality and to set things right you have to expend double the effort than originally enivsaged.
you should read about the limitations of DPT. if the weld is critical dpt is not sufficient. DPT is a quick way of finding if the wled is really bad, but it will not catch all the defects.