Rub as much of the rust off with steel wool, a fine grade or perhaps one of the 3M Scotch-Brite sanding thingies. I prefer plain steel wool for this job because it cuts better. All you want to do is eliminate the rust. So rub in all directions till most rust is gone. If you have some canned air, hold it upside down and blow off the dust, gently. You can repaint with either a water based or regular enamel. If you plan to keep it white I’d wipe it on just the grill. If you want to change colors you could spray the sides and then spray a pad and wipe on the grill.