Removing all existing paint & coatings is generally the best method of preparation. At a minimum, loose, cracked or chipping paint must be removed. 

Also be sure to use proper protective equipment when removing paint. It is possible when working on historic metal roofs that lead based paint will be encountered. All regulations and safety procedures concerning lead-based paint must be observed. The Department of Interior has published Preservation Brief #37 - Appropriate Methods for Reducing Lead Paint Hazards in Historic Housing which contains helpful information on addressing the problem of lead based paint on historic structures. The Environmental Protection Agency also recently issued the Renovation, Repair and Painting Rule which is aimed at preventing lead poisoning. Homeowners, and contractors especially, should familiarize themselves with these briefs and rules of law.