MIG Wire Finder

Both are sold as ER70S-6 under the American standard. In Europe, SG2 (G3Si1) is the everyday wire for mild steel. SG3 (G4Si1) has a little more manganese and silicon, which clean up light mill scale and give a slightly more fluid pool and higher strength. For general fabrication either works; choose SG3 on steel that is not perfectly clean or when you run higher currents on thicker plate. Copper-free versions of both feed well and cut contact tip wear.

Gas-shielded solid wire needs a gas bottle and gives clean welds with little spatter and no slag, so it is the choice indoors. Gasless flux-cored wire makes its own shield from the flux inside, so there is no bottle to hire and a breeze will not blow the shield away; it is good outdoors. The trade-off is more spatter and slag to chip off. Gasless wire runs with the torch on negative polarity (DCEN), so swap the leads on your machine before you start.

Match the filler to the steel. 308L suits 304 and 304L, the most common stainless. 316L adds 2 to 3% molybdenum for better resistance to salt and chemicals, so use it on 316; it is also fine on 304. The reverse is not true: 308L on 316 leaves a weak spot for corrosion. 309L is over-alloyed for joining stainless to mild steel, where the weld mixes with the carbon steel side. The "L" means low carbon, which stops the weld area rusting at the grain boundaries.

5356 (AlMg5) is the all-rounder for 5xxx and 6xxx sheet and extrusion: strong, stiff enough to feed well, and it stays a good colour match after anodising. 5183 (AlMg4.5Mn) is stronger again and is the choice for 5083 marine plate. 4043 (AlSi5) flows more easily and resists hot cracking, which suits castings and some 6xxx work, but it turns dark grey when anodised. Never use 4043 on 5xxx alloys with more than 2.5% magnesium: the weld can become brittle.

MIG wire comes on three spool sizes, named by their outside diameter. 100mm spools hold 0.5 to 1kg and fit most small hobby welders. 200mm spools hold 4 to 5kg of steel and fit most mid-size machines. 300mm reels hold 12.5 to 18kg and suit workshop machines, with the lowest price per kg. Aluminium is lighter, so the same spool holds less: about 0.5kg, 2kg and 6 to 7kg. Measure the spool holder in your machine before you buy.