Sandblasting sheet metal without messing it up is an art.... lol.... I'd take it someplace and have it done. Then maybe have the underside rubber coated. I have a small pressurized sand blaster. Its fine for small parts but a pia on about anything else, lol. If you don't already have one a big compressor makes life easier, lots of air tools, big hammers
, a fridge to keep beverages close, and ya can't go wrong with sata spray gun. Good luck
