For most mild steel welding, ER70S-6 is the go-to filler rod. It works across TIG, MIG, and other gas-shielded processes, handles light surface contamination well, and produces strong, clean welds on the carbon steel you’re most likely working with. But the best choice depends on your welding process, the condition of your material, and the thickness you’re joining.
ER70S-6: The Standard Choice
ER70S-6 is the most widely used filler rod for mild steel, and for good reason. It contains higher levels of silicon and manganese, which act as deoxidizers to help the weld puddle flow smoothly and resist porosity. The “70” in its name refers to a minimum tensile strength of 70,000 psi, though in practice Lincoln Electric data shows these wires typically deliver 80,000 to 90,000 psi tensile strength and 65,000 to 75,000 psi yield strength. That’s more than enough for any mild steel application.
The extra silicon in ER70S-6 makes it more forgiving on material that has a thin layer of mill scale or light surface oxidation. It wets out better than lower-silicon alternatives, giving you a smoother bead with good tie-in at the toes. If you’re buying one rod to keep on the shelf for general mild steel work, this is it.
ER70S-2: For Cleaner, More Critical Welds
ER70S-2 is a triple-deoxidized wire containing zirconium, titanium, and aluminum on top of the standard silicon and manganese. Those extra elements scavenge oxygen from the weld pool more aggressively, which produces an exceptionally clean weld with minimal porosity. This makes ER70S-2 the preferred choice for open-root pipe welds, pressure vessels, and any joint where weld quality is critical.
The tradeoff is cost. ER70S-2 is noticeably more expensive than ER70S-6, and for typical fabrication work like brackets, frames, and structural pieces, you won’t see a meaningful difference in the finished weld. Save ER70S-2 for jobs where code requirements or joint integrity demand it, or when you’re TIG welding thin material and want the cleanest possible puddle.
Choosing the Right Rod Diameter
Filler rod diameter should roughly match the thickness of the material you’re welding. Going too large makes it hard to control heat input on thin stock, while going too small on thick plate means you’ll spend forever building up your weld. Here’s a practical starting point for TIG welding mild steel:
- 1/16″ rod: sheet metal and material up to about 1/16″ (1.6 mm) thick
- 3/32″ rod: material from 1/16″ to roughly 3/16″ (1.6 to 5 mm) thick
- 1/8″ rod: material over 3/16″ (5 mm) thick
These aren’t rigid rules. Many welders prefer to size down one step for better puddle control, especially on fillet welds where you can make multiple passes. For MIG welding, wire diameter is a separate consideration driven by your machine’s feed system, with .030″ and .035″ being the most common for mild steel in the hobbyist and light fabrication range.
Welding Over Mill Scale or Rust
Clean material always produces better welds, but real-world steel often comes with mill scale, light rust, or surface contamination. How you handle this affects which filler metal works best.
Solid wires like ER70S-6 can tolerate light mill scale, but they don’t wet out as well on contaminated surfaces. You’ll need to slow your travel speed to compensate, and heavy scale or rust will still cause problems. If you can’t grind or wire-brush the joint area clean, metal-cored or flux-cored wires are better options. Metal-cored wires can be formulated with more deoxidizers than solid wire, so they handle surface impurities at higher travel speeds. For material with heavy mill scale or rust, flux-cored options like E70T-1 (for flat and horizontal positions) or E71T-1 (for all-position welding) provide the best results, with good bead appearance even on dirty material.
The bottom line: grind your joints when you can. When you can’t, step up to a cored wire rather than fighting a solid wire on contaminated steel.
Filler Rods for Oxy-Acetylene Welding
If you’re gas welding mild steel with an oxy-acetylene torch, the filler rod classification system is different. The two main options are RG45 and RG60, both covered under AWS A5.2.
RG45 has a minimum tensile strength of 45,000 psi and is designed for sheet metal, plate, and pipe where high strength isn’t required. It flows easily and works well for general repairs and light fabrication. RG60 is a high-strength rod with a minimum deposit strength of 60,000 psi, intended for castings, structural pipe, and shapes where the joint needs to match or approach the base metal’s strength. For most hobby and repair work with an oxy-acetylene torch, RG45 is the practical choice.
Storing Your Filler Rods
Mild steel filler rods will rust if stored in damp conditions, and surface oxidation on the rod transfers directly into your weld as contamination. Keep rods in their original sealed packaging until you need them. Once opened, store them in a dry location away from concrete floors (which wick moisture). A sealed tube or plastic bag works fine for bare TIG rods and MIG wire.
Coated stick electrodes require more care. Moisture in the flux coating causes porosity, cracking, and hydrogen embrittlement. Opened containers of low-hydrogen electrodes should be kept in a rod oven at 250 to 300°F. Standard mild steel stick electrodes (like E6013 or E7018) are less sensitive but still benefit from dry storage. Avoid using longer drying times or higher temperatures than recommended, as excessive heat can damage the electrode coating.
Quick Selection by Welding Process
- TIG welding: ER70S-6 for general work, ER70S-2 for critical or code work
- MIG welding: ER70S-6 solid wire for clean material, E70T-1 or E71T-1 flux-cored wire for dirty or rusty steel
- Stick welding: E6013 for thin material and easy striking, E7018 for structural work requiring low hydrogen and higher strength
- Oxy-acetylene: RG45 for general work, RG60 for structural or high-strength applications
All of these classifications fall under the American Welding Society’s A5 series of filler metal specifications. The “ER” prefix means the wire can be used as either an electrode (in MIG) or a rod (in TIG). The “70” indicates 70,000 psi minimum tensile strength. The “S” means solid wire, and the number after it (like 2 or 6) identifies the specific chemical composition. Once you understand that naming system, picking the right rod for any mild steel job becomes straightforward.