If in doubt, coat the edge with the proper paint. The flatter the steel application, the more it will weather. Sidewalls take the least abuse, roofs with little pitch take the most. Never use questionable steel on a roof with a shallow pitch. Steel that has been scratched, water spotted, etc.
Water build up from condensation, moisture and rain is bad for the coatings and paint on steel.
Moisture laying on steel in storage is the cause of white rust. It is recommended that steel roofing be put up immediately upon delivery. Panels should be unbanded and stood on end against an interior wall with the bottom ends spread out, and kept off the ground on blocks. If moisture is found, dry each panel immediately and thoroughly. It is highly recommended that steel roofing never be stored outside.