[Jrainman]

wash the sheetmetal with water born degreaser to get the oils out ,after you dry it you can sand it to create a tooth for the epoxy to adhere ,but you can also use vinegar instead of sanding ,the vinegar is a acid and will create etching in the sheetmetal so the epoxy primer will adhere, this method has been used for a very long time . leaving it out side will will do nothing but make it in to a used old sheet, witch will eventualy wear the zinc in the sheet away and after a point in time the sheet will start rusting .