Best MIG Wire for Welding Over Rust and Mill Scale
Every fab shop has the same problem: the steel on the rack is never perfectly clean. Hot-rolled plate arrives with blue-grey mill scale, and bar stock that sat by the door picks up surface rust. If you are running solid MIG wire, the wire you choose decides how much of that contamination your weld can tolerate before porosity shows up.
Why ER70S-6 Is the Usual Answer
ER70S-6 (AWS A5.18) carries higher levels of the deoxidizers manganese and silicon than lower-alloyed solid wires. During welding those elements bind with oxygen in the weld pool and float out as a thin silicate island on the bead, which is what lets the wire weld cleanly over light rust and mill scale while still producing sound, porosity-free metal. It is a 70,000 psi tensile class wire, runs on DC+ (reverse polarity), and works with 100% CO₂ or 75–80% argon / 20–25% CO₂.
Typical chemistry for HYW ER70S-6 is 1.40–1.85% manganese and 0.80–1.15% silicon. That extra deoxidizer content is the reason shops standardize on it for general fabrication.
Light Rust vs. Heavy Rust: Know the Limit
"Tolerant" does not mean "immune." No solid wire turns flaking, pitted, or heavily scaled steel into a good joint. Use this as a rule of thumb:
| Surface condition | What to do |
|---|---|
| Tight mill scale, intact | ER70S-6 will usually weld through it; clean the joint edges for best results on critical or structural joints |
| Light surface rust | Wire brush or flap-disc the joint area, then weld with ER70S-6 |
| Loose, flaking, or pitted rust | Grind to bright metal; consider a flux-cored wire for dirty field repairs |
| Oil, paint, galvanizing, moisture | Remove completely; these cause porosity and fume problems no wire can fix |
Practical Tips When the Steel Is Not Clean
- Clean a strip along the joint. Even an inch on each side of the weld removes most of the contamination that matters.
- Keep the wire itself dry. Rust on the wire is worse than rust on the plate. Store spools and drums in a dry location.
- Mind your gas. Pure CO₂ gives deeper penetration and handles dirty steel well; 75/25 argon blends give a smoother, lower-spatter arc. Both are approved for ER70S-6.
- Watch the tip and liner. Scale and rust particles carried into the gun accelerate wear, so check consumables more often on dirty jobs.
- Adjust travel speed. Slow slightly to give the pool time to gas out and tie in.
Where This Matters Most
- General fabrication and structural steel
- Heavy equipment, trailers, and farm machinery repair
- Hot-rolled plate and bar stock used as-received
- High-volume production where grinding every joint is not practical
Sizes and Starting Amperage
HYW ER70S-6 in the 550 lb drum is offered in .035" (general guideline 50–180 A) and .045" (75–350 A). Actual settings depend on thickness, joint design, gas, and position, so treat these as ranges and tune on scrap first.
Buying: Spools vs. Drums
For occasional repair work, spools are the easy choice. If your shop runs wire every day, or you feed an automated or robotic cell, the 550 lb drum cuts down on spool changes and lowers cost per pound. Contact us for pallet and distributor pricing.
Ready to stock up?
Shop the ER70S-6 550 lb drum or browse the full ER70S-6 MIG wire collection.
FAQ
Can I weld over rust with MIG wire?
Light surface rust and tight mill scale are manageable with ER70S-6 thanks to its deoxidizers, but heavy or flaking rust should be ground off. Cleaner steel always gives a stronger, better-looking weld.
Do I need to remove mill scale before MIG welding?
For non-critical work, ER70S-6 can often weld through tight mill scale. For structural, code, or load-bearing joints, clean the joint area to bright metal.
Which shielding gas is better on dirty steel?
100% CO₂ penetrates deeper and is generally more forgiving; 75/25 argon/CO₂ offers a smoother arc with less spatter. Both are suitable for ER70S-6.
What if the steel is too rusty for MIG?
Grind to bright metal, or consider a flux-cored wire for field repairs where full prep is not possible.
Safety: Welding produces fumes, arc radiation, and sparks. Use proper ventilation, a welding helmet, gloves, and protective clothing, and follow your employer's and the manufacturer's safety instructions. Cleaning rusty or coated steel can release hazardous dust and fumes; wear appropriate respiratory protection.