For SMAW or "stick" welding, DC or AC can be used. Most common electrode types perform best with DC electrode negative, or "DCEN". (The exception is E7024 which I feel works slightly better with AC, though it works well enough with DCEN.) Most common electrodes can be used with AC as well. The difference is that AC will generally produce a wider, flatter bead, with less penetration. This can be a benefit if you are welding thicknesses under 3mm, AC delivers more heat energy into melting the electrode and less into the melting the base metal. Electrodes tend to perform poorly and erratically using DC electrode positive (DCEP) I wouldn't advise using this. Some SMAW welding machines you can switch between AC and DC. Some are AC only....... Some are DC only.AC only machines are usually very simple and robust. they are usually low cost. .................................. GMAW, or "MIG" welding always uses DCEP! There is no reason to use either AC or DCEN for GMAW, as these will cause weld defects. In any case, GMAW requires a special "constant voltage" power source, whereas SMAW and GTAW require "constant current."...... Power sources for SMAW should never be used for GMAW. ..................................... GTAW, or "TIG" welding usually uses DCEN, or AC. DCEN is for welding steel, stainless, and nickel alloys, I believe. AC is used for welding aluminum. In addition to AC, Most GTAW machines also have a "high frequency" circuit which is also used with AC when welding aluminum. ................................. Gouging is used to remove welds, plain and simple..... Like with SMAW welding, arc gouging works best with DCEN. In order to do gouging, you also need an air compressor and a special electrode holder that has an air nozzle in it. Also, a small AC machine probably wont have enough power to do gouging. Gouging requires more current than welding; you'd need a medium-duty or heavy-duty machine