MIG Porosity on Mild Steel With Mill Scale or Light Rust? Start With Wire Choice (and Fix the Real Cause)
Porosity is one of the most expensive small weld defects. You can run a bead that looks acceptable from five feet away, then grind it and find pinholes all the way through. The job slows down fast: rework, extra consumables, and lost confidence in your settings.
Across MIG troubleshooting threads, the same pattern shows up: porosity that appears on mild steel with mill scale or light rust is often a combination of contamination and inconsistent shielding. You can fix a lot of this with better prep and gas delivery, but one lever that gets overlooked is wire selection. ER70S-6 is commonly chosen because it is designed to be more forgiving on less-than-perfect steel than some other mild-steel wires.
Featured Product Quick Take
- Name: Washington Alloy 33 Lb Mig Welding Wire ER70S-6 .030 For X-Ray Quality Welds
- SKU: E70S6E8 - 1 EACH
- Price: Unknown (Verify)
- What it fixes: MIG porosity on mild steel when the base metal has mill scale, light rust, or other surface contamination that makes shielding and puddle chemistry less forgiving.
- Why it matters: porosity is rework: grinding, re-welding, and lost time. A more forgiving wire can reduce scrap and help you get consistent results faster (without pretending prep does not matter).
- •Brand: Washington Alloy (product page)
- •AWS classification: ER70S-6 (product page)
- •Wire diameter: .030 (product title)
- •Spool weight: 33 lb (product title)
- •Price: Unknown (Verify)
Wire diameter must match drive rolls, liner, and contact tip. If you are unsure, call with machine model + process + thickness before ordering.
What This Fix Solves
- Porosity that shows up on mill scale, light rust, or shop-worn material
- random pinholes that come and go even when settings look stable
- Beads that look fine until you grind or bend-test them
- Rework loops caused by chasing voltage and wire speed instead of root cause
Root Cause Breakdown (Why Porosity Shows Up on Mild Steel)
- Surface contamination: mill scale, rust, paint, oil, and cutting fluids can release gas into the puddle.
- Shielding gas coverage problems: leaks, low flow, or turbulence can pull air into the arc zone.
- Drafts: fans, open doors, and air movement disrupt shielding (especially with smaller nozzles).
- Consumable fitment issues: wrong tip size, liner drag, or drive roll slip can cause an unstable arc and inconsistent puddle behavior.
- Wire/base-metal mismatch: some wires are less forgiving when the base metal is not perfectly clean. Wire choice is not a substitute for prep, but it can widen your process window.
The Fix (Actionable Steps)
- Prep the weld zone: remove paint/oil and knock down heavy rust. If you cannot prep perfectly, at least clean the immediate weld path.
- Leak-check your gas system: cylinder rregulator rhose rmachine. Use an approved leak-detection solution.
- Set a stable, repeatable flow: use a flow gauge/flowmeter style regulator so you are not guessing. Avoid excessive flow that creates turbulence.
- Confirm wire fitment end-to-end: .030 wire must match drive rolls, liner, and contact tip. A mismatch creates feed drag and arc instability.
- Switch to ER70S-6 when appropriate: if you are welding mild steel that is not perfectly clean, ER70S-6 is commonly selected for its deoxidizer package (verify for your application and code requirements).
- Run a controlled test bead: same joint, same technique. If porosity persists, stop and isolate: gas coverage vs contamination vs machine issue.
Note: We are not publishing fixed voltage/WFS numbers here because they are machine- and joint-dependent. The goal is to eliminate root causes first.
Key Specs / Fitment Notes (Bullets Only)
- SKU: E70S6E8 - 1 EACH (product page)
- Wire type: ER70S-6 (product page)
- Diameter: .030 (product title)
- Spool weight: 33 lb (product title)
- Shielding gas recommendation: Unknown (Verify)
- Polarity: Unknown (Verify)
Before You Order Checklist
- Machine: welder make/model + feeder model (if separate)
- Process: MIG (solid wire)
- Material: mild steel (clean vs mill scale vs light rust)
- Thickness: typical range you weld
- Consumables: .030 contact tip + matching liner + matching drive rolls
- Torch/gun: gun model + consumable family
- Gas: C25 vs 100% CO2 vs other mix (verify)
If you want us to confirm fitment fast, email the wire SKU + your machine model + wire diameter + typical thickness.
Recommended Accessories (Priority Order)
Comparison Block (Alternatives)
Common choice for mild steel. Often selected when you need a wire that is more forgiving on mill scale or less-than-perfect surfaces (prep still matters).
Often used on cleaner steel. If you are seeing porosity on dirty material, the fix may be prep, gas delivery, or switching wire type (verify for your application).
Usually wastes time. Porosity is typically gas coverage, contamination, or base-metal condition not a small voltage tweak.
Safety Note
Welding fumes and grinding dust are hazardous. Use appropriate ventilation and PPE, and follow your equipment manual. Shielding gas can displace oxygen in confined spacessecure cylinders and leak-check connections. Do not bypass safety devices.
Add to Cart or Confirm Fitment First
Add the Washington Alloy ER70S-6 .030 (33 lb) to your cart. Not sure if .030 is right for your feeder and gun? Call with machine model + process + thickness and well match the correct wire and consumables.