Spray painted Folding chairs

I took a big leap and painted 13 old metal folding chairs.  I use these all the time for family gatherings and was tired of the white, rusted appearance of these chairs.  I love how the chairs turned out using the hammered bronze/brown paint.  However, the paint chips off fairly easily.  I just didn't think this would happen.  I cleaned the chairs well, but because they were no longer shiny and slick, didn't sand them - is that the problem?  Or, would they have chipped anyway?

Thanks for your thoughts.