Polarity is ALWAYS DCEP for GMAW and gas-shielded FCAW (aka "dual-shield"). In general, the best wire diameter for spray transfer MIG welds on parts greater than 5 mm thickness is 1.2mm (0.045 inch) wire. The optimum spray-transfer current that provides ideal weld puddle control with the 1.2mm MIG wire is typically found between 250 and 350 amps. Wire feed speed is around 450-550 in/min, or around 1.3 meters/min. Voltage should be between 28 and 31 volts. Once your feed speed is set, adjust the voltage up or down until you hear the proper spray transfer sound- a nice, soft "crackle." Note that at this thickness of material burn through is not much of a concern for flat and horizontal welds. From a standpoint of weld quality and productivity, your goal should be to increase both the weld current and the wire feed speed to the greatest levels that will still produce a weld puddle that is easy to control. With spray transfer welds the contact tip should be recessed about 5-9mm inside the nozzle. To a point, spray transfer benefits from longer wire stick-out. Note that spray transfer only produces a controllable puddle on flat and horizontal welds! For out-of-position welds, consider using an all-position, gas shielded, 1.2mm E70-T1 FCAW wire. I reccomend ESAB or Kobleco wires. Unlike solid GMAW wires, wire chemistry and flux composition can vary dramatically between different FCAW wire manufacturers. Keep in mind that buying bargain priced wires form no-name manufacturers will not result in cost reductions- when the cheap, dirty wires are the cause of serious weld quality issues.