In order to get the strength needed to carry roof loads, the flat sheets are run through a roll forming machine.  In order to form the ribs, the steel is stretched and bent.  This stretching and bending puts cracks in the paint and zinc, particularly in polyester paint applied over full hard steel.  These cracks are not visible to the naked eye, but they can be clearly seen under a microscope.  These cracks expose the steel and zinc to the moisture and pollutants in the atmosphere, and this is where red rust begins.

The Morton, more flexible commercial quality steel with a heavier zinc coating and more flexible KYNAR 500^® paint resists cracking during the roll forming operation.