I agree with Greg. The less time spent on the roof is better for the roof. If your main goal is to have an overhang or add insulation with your new roof, then you would need a need roof. But if the goal is to not leak, I feel that you maintain the original roof as long as possible. When the repairs become too frequent or big, then it may be time to look at a new roof.
All factory metal roofs are going to have a finite usable lifetime. Eventually, rust and or a build-up of sealants is going to make resealing the roof too expensive or too much work. Anything you squirt out of a tube or paint out of a bucket will also have a limited lifespan. They will all deteriorate and fail eventually. So my thinking is to make each reseal or recoat last as long as possible to minimize the wear and tear on the roof. That is why I only recommend doing the recoats properly and not taking short cuts.
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All information and advice given is for entertainment and informational purposes only. The person doing the work is solely responsible to insure that their work complies with their local building code and OSHA safety regulations.