Don't use glass beads except for hard items like brackets, hood hinges, trunk hinges, etc, sheet steel reacts to glass bead regardless of air pressure, the metal will swell were struck with the bead abrasive,.....sand is prefered, but messy to remove from all the crevises,.....I'd recommend "black beauty" silicate abrasive, "medium grade", and blast any sheetmetal at a 45 degree angle, and do not concentrate the media on any one area too long, heat is one of your enemies, the constant blasting is equivalant to "Pick hammering" it will affect the area if you stay on it to long!.....the black beauty "rolls" out of the crevises when you blow down the car after, sand just lays there