Metal roof leaks are easier to solve than their membrane counterparts. Even if the metal sheet is rusted, as long as it has not rusted through, the sheet itself is 100% waterproof. Now you only need to concentrate on seams, screws and penetrations.
Metal roofing profiles with exposed fasteners (corrugated and r-panel) have the highest tendency for leaks during its first 5 years. Lexis Coatings’ case study of metal roof leaks discusses the accumulation of dirt along the seams that cause 60% of roof leaks involving these profiles.
Bottom line is, once you have determined the source of the leak, always apply seam sealers from the highest point which is the ridge cap or flashing down to the gutter. If you start from the gutter and seal halfway up the roof or seal in short segments where you feel the leak is occuring, it will aggravate the leak.