what is media blasting?

Media blasting, commonly referred to as sand blasting, is an abrasive media that is fed through a nozzle along with pressurized air. 

why does media blasting need to be done?

Media blasting is done prior to coating to remove any old coatings, rust, and contaminants on the surface.  It also etches the surface of the substrate and preps it for coating. The blasted surface gives the coating something to adhere to and insures durability