Telling a good weld from a bad weld can mean the difference between a structure that lasts decades and one that fails under the first real load. Most people can feel when something looks wrong — but they can’t name the defect, understand what caused it, or know what to do next.
This guide changes that. By the end, you’ll be able to diagnose any weld defect by sight, understand the root cause, and take the right corrective action — whether you’re a welder, an engineer, or a buyer evaluating a supplier’s work.This is especially critical in sheet metal fabrication, where weld quality directly determines part dimensional accuracy.
Good Weld vs Bad Weld — The Instant Visual Reference
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A good weld has a uniform, consistent bead with smooth ripples, full penetration into the base metal, no visible porosity or cracks, minimal spatter, and clean tie-ins at both weld toes. A bad weld shows irregular bead shape, surface porosity, undercut grooves, heavy spatter, visible cracks, or lack of fusion — any of which can compromise structural integrity.
| Feature | ✓ Good Weld | ✗ Bad Weld |
| Bead shape | Uniform width, smooth ripples, consistent height | Wavy, irregular, piled high or sunken |
| Surface texture | Smooth with tight, even ripple pattern | Rough, pitted, or lumpy surface |
| Color (steel) | Light gold/straw to light blue at edges | Black, dark grey, or heavy bluing = overheating |
| Penetration | Full fusion into both base metal pieces | Sits on top; no bonding below the surface |
| Spatter | Minimal to none around the joint | Heavy metal droplets scattered around bead |
| Cracks | None visible on surface or at toes | Visible cracks along bead, crater, or HAZ |
| Undercut | Clean, flat tie-in at both weld toes | Groove or notch cut into base metal edge |
| Porosity | Solid, continuous bead — no holes | Pinholes or gas pockets visible in bead |
| Overlap | Bead blends smoothly into base metal | Bead rolls over edge without fusing (cold lap) |
| Distortion | Base metal stays flat and square | Metal warped, bowed, or pulled out of alignment |
| Overall appearance | Clean, neat — looks intentional and controlled | Inconsistent — looks hurried or uncontrolled |
Use this table on the shop floor. If any row in the “Bad Weld” column describes what you’re looking at, keep reading — the defect-by-defect breakdown below will tell you exactly what went wrong and how to fix it.
What Makes a Good Weld? The 5 Non-Negotiables
Every weld that passes inspection — whether for a car frame, a pipeline, or a structural column — shares five core qualities. These aren’t aesthetic standards. They’re structural requirements.
- Full penetration. The filler metal must fuse with both base metal pieces through the full joint thickness. A weld that sits on the surface without bonding below has near-zero structural value. Check the root side of butt welds — you should see a slight bead flush on the back face.
- Consistent bead geometry. Width, height, and ripple spacing should be uniform from start to finish. Variation means the welder’s travel speed or arc length changed — and so did the heat input and penetration depth at each point.
- Clean fusion zone with no inclusions. The boundary between weld metal and base metal should be continuous and bonded. Porosity, slag inclusions, or cold lap at this boundary creates stress concentration points where cracks initiate.
- Correct profile for the joint type. Fillet welds should be slightly concave or flat — a convex fillet concentrates stress at the toes. Groove welds need to be filled flush or slightly proud. Profile requirements are set by the weld specification (AWS, ISO, etc.) for the application.
- No heat damage to parent metal. The heat-affected zone (HAZ) should show a controlled color gradient — light straw to gold on steel is normal. Black or heavily blued HAZ means overheating, which changes the grain structure and reduces strength in the surrounding base metal.
8 Types of Bad Welds — Visual Signs, Causes, and Fixes
Each defect below is rated by severity: COSMETIC (appearance only) · STRUCTURAL (reduces load capacity) · CRITICAL (risk of catastrophic failure). This is the ranking no competitor article gives you.
1. Porosity
Severity: STRUCTURAL
What it looks like: Pinholes, pits, or a spongy-looking surface on the weld bead. Subsurface porosity may not be visible without NDT — but surface porosity always signals more voids beneath.
What caused it: Contamination of the weld pool: oil, moisture, mill scale, or rust on the base metal or filler wire. Inadequate shielding gas flow, a leaking gas hose, or welding outdoors in wind that blows the gas shield away.
How to fix it: Grind out the affected section completely, clean the base metal with acetone or a stainless wire brush, check shielding gas flow rate and hose connections, then re-weld.
2. Undercutting
Severity: STRUCTURAL
What it looks like: A groove or notch cut into the base metal running along one or both weld toes. Looks like the base metal has been eroded right where the bead meets the parent metal.
What caused it: Too much amperage, too fast travel speed, or incorrect electrode angle directing the arc toward the weld toe instead of the joint center. Common in vertical and overhead welding positions.How to fix it: Keep undercut depth under the code-specified limit (typically 0.8mm for structural welds). Deep undercut requires grinding to blend the groove smoothly, then applying a repair bead at lower heat input.
3. Incomplete Fusion / Cold Lap
Severity: CRITICAL
What it looks like: The bead surface looks acceptable but the weld has not actually bonded to the base metal at the sidewall or root. Often invisible on the surface — detected by bend test or ultrasonic inspection.
What caused it: Travel speed too fast, amperage too low, or wrong electrode angle. The weld pool moves ahead of the arc before proper fusion occurs. Particularly common in aluminum MIG welding.
How to fix it: This defect cannot be fixed by adding more weld on top — the trapped unfused layer remains. Grind the entire weld out down to solid base metal, adjust settings, and re-weld with verified parameters.
4. Cracks
Severity: CRITICAL
What it looks like: Visible linear fractures along the weld bead, in the crater at the end of the weld, or in the heat-affected zone beside the bead. Hot cracks appear during cooling; cold cracks appear hours or days later.
What caused it: Hot cracks: high sulfur or phosphorus in the base metal, too concave a bead profile, or rapid cooling. Cold cracks: hydrogen contamination from moisture on electrode coatings or base metal, high restraint, or incorrect pre-heat on hardenable steels.
How to fix it: Crack repair is non-negotiable — grind completely past the crack tip (confirmed by dye penetrant), pre-heat if required by material specification, and re-weld with low-hydrogen consumables. Never weld over a crack.
5. Excessive Spatter
Severity: COSMETIC
What it looks like: Large metal droplets scattered across the base metal surface around the weld bead, some fused to the surface, others sitting loosely.
What caused it: Voltage too low or too high relative to wire feed speed in MIG welding. Contaminated wire or base metal. Incorrect shielding gas mixture — CO2-rich gas produces more spatter than Ar/CO2 blends.
How to fix it: Adjust voltage and wire feed speed to the correct relationship for the wire diameter and metal thickness. Clean base metal before welding. Cosmetic spatter can be ground or chipped off — structural quality is not affected if the weld itself is sound.
6. Burn-Through
Severity: CRITICAL
What it looks like: A hole completely melted through the base metal. On thin sheet metal, the surrounding area may show a large heat discoloration halo.
What caused it: Amperage far too high for the material thickness, travel speed too slow, or attempting to weld over a gap that is too large. Very common when welding thin gauge sheet metal without a backing bar.
How to fix it: Stop and allow the part to cool. For small burn-through, a copper backing bar can be placed behind the hole and the area filled with careful short-arc technique. Large burn-through in a structural joint typically requires cutting out a section and inserting new material.
7. Distortion / Warping
Severity: STRUCTURAL
What it looks like: The base metal has bent, bowed, or twisted out of its original flat or square alignment after welding. The weld itself may look fine, but the part geometry is no longer correct.
What caused it: Uncontrolled heat input and thermal expansion. Welding one side of a joint continuously causes that side to shrink on cooling and pulls the part toward the weld. Common in thin plate, long seam welds, and multi-pass welds.
How to fix it: Prevention is always better than correction. Use staggered backstep sequences, clamp parts in fixture before welding, balance heat by alternating weld sides. Distorted parts can sometimes be straightened by applying heat with a torch to the opposite side — but this requires skill and is not always possible.
8. Overlap / Cold Lap
Severity: STRUCTURAL
What it looks like: The weld bead rolls over the base metal edge without fusing to it — the bead sits on top of the surface with a sharp, notch-like boundary at the toe instead of blending smoothly.
What caused it: Travel speed too slow, allowing the weld pool to flow ahead of the arc. Electrode angle too flat. Common in horizontal fillet welds where gravity pulls the pool down-slope.
How to fix it: Grind the overlapping bead edge to remove the notch, verify correct electrode angle and travel speed, and apply a repair pass with the arc targeted precisely at the toe line.
Good vs Bad Welds by Welding Process
Different welding processes produce different visual signatures — both when done well and when done poorly. Here’s what to look for in each of the four most common processes.
MIG Welding (GMAW) — Most Common Process
If you’re unsure which process suits your application, our MIG vs TIG welding guide covers the decision criteria in detail.
| Good Weld Signs | Bad Weld Signs |
| Uniform bead width, end to end | Wavy, inconsistent bead width |
| Smooth, consistent ripple pattern | Rough or pitted bead surface |
| Flat or slightly convex profile | Heavy spatter on surrounding metal |
| Light gold/straw color on base metal | Undercut grooves at weld toes |
| Minimal spatter, clean joint area | No discoloration on base metal (cold weld) |
| Solid bead — no pinholes or pits | Irregular bead height — piled in sections |
TIG Welding (GTAW) — Highest Precision
| Good Weld Signs | Bad Weld Signs |
| Stacked dime pattern: uniform, tight rings | Flat, wide beads — no distinct dime pattern |
| Bright, shiny, silver surface (steel) | Grey, dull, or black surface (overheating) |
| No spatter of any kind | Tungsten inclusions (grey specks in bead) |
| No visible tungsten contamination | Porosity from contaminated filler rod |
| Clean tie-in at both toes | Burnout and undercut at edges |
| Narrow, neat bead — controlled heat | Inconsistent bead width — wavering arc |
Stick Welding (SMAW) — Heavy Fabrication
| Good Weld Signs | Bad Weld Signs |
| Uniform bead with consistent slag coverage | Slag inclusions trapped in bead (visible pits after chipping) |
| Clean bead after slag removal — no trapped slag | Crater cracks at weld end — not filled |
| Even crater fill at end of each pass | Highly irregular bead from arc wander |
| Correct width for electrode diameter | Excessive undercut from too-high amperage |
| Smooth, controlled ripple pattern | Heavy spatter from wet or old electrodes |
| No visible cracks in crater or bead | Cold lap from restarting without cleaning slag |
Flux-Core (FCAW) — High Deposition Production
| Good Weld Signs | Bad Weld Signs |
| Consistent, full-coverage slag that releases cleanly | Slag that sticks or won’t release cleanly (wrong polarity) |
| Solid bead beneath slag — no porosity | Worm-hole porosity beneath slag surface |
| Uniform bead profile, correct for wire diameter | Irregular bead from wire feed inconsistency |
| Smooth weld toes with no undercut | Excessive spatter from wrong voltage setting |
| Appropriate spatter level for gas/no-gas process | Undercut at toes from high travel speed |
| Consistent penetration across full joint length | Cold lap from wire speed too high for amperage |
How to Inspect a Weld — The 3-Level Check
Knowing what bad welds look like is only half the job. The other half is having a systematic inspection routine. Here’s a three-level process any shop can run without specialist equipment.
Level 1: Visual Inspection (VT)
Start at the right distance and angle. Position yourself 300–600mm from the weld with a flashlight or inspection light held at a low, raking angle — this creates shadows that reveal surface irregularities invisible under direct light.
Work systematically: check the full bead length for width consistency, inspect both toes for undercut, scan the bead surface for porosity, and check start and stop points for crater cracks. A 10x magnifying loupe will catch hairline cracks the naked eye misses.
Level 2: Tactile Check
Run a wire brush firmly along the full weld bead — this removes loose slag and exposes any surface porosity or inclusions underneath. After brushing, run a fingertip along both weld toes feeling for the undercut groove that eyes sometimes miss.
For production welding, a standard weld gauge (fillet weld gauge or hi-lo gauge for groove welds) quickly confirms bead size meets specification without measurement ambiguity.
Level 3: Destructive Testing (for Critical Joints)
The fillet break test is the standard shop-floor destructive test. A sample fillet weld coupon is bent or hammered until the joint breaks — the fracture face reveals penetration depth, fusion quality, and internal porosity that no surface inspection can find.
The bend test applies to groove welds: a coupon is face-bent and root-bent over a mandrel. Cracks longer than 3mm on the tension surface indicate a reject. Beyond these shop tests, structural and safety-critical welds require NDT — ultrasonic testing (UT) for internal defects, dye penetrant (PT) for surface cracks in non-magnetic materials.
What Causes Bad Welds? Root Causes by Category
Bad welds don’t happen randomly. Every defect traces back to one of three root cause categories. Fixing the right category prevents recurrence.
Operator Factors
- Wrong travel speed: too fast = lack of fusion, too slow = burn-through or overlap
- Wrong arc length / standoff: too long = porosity and spatter; too short = erratic arc and tungsten contamination in TIG
- Poor joint preparation: leaving rust, mill scale, oil, or moisture on the base metal causes porosity regardless of machine settings
- Incorrect electrode angle: wrong work angle or travel angle directs the arc into the base metal instead of the joint, causing undercut or cold lap
- Fatigue and inconsistency: long production runs without breaks produce welds that deteriorate in quality as the session continues
Equipment Factors
- Wrong amperage or voltage settings: the single most common machine-side cause of all defects from porosity to burn-through
- Incorrect wire feed speed (MIG/FCAW): too fast creates cold lap and spatter; too slow creates burn-back and arc instability
- Contaminated or low shielding gas flow: leaks, clogged diffusers, or incorrect flow rate expose the weld pool to atmospheric contamination
- Worn contact tips or liners (MIG): causes inconsistent wire feed and arc instability, visible as irregular bead width
Material Factors
- Contaminated base metal: the number-one cause of porosity — clean thoroughly with acetone or dedicated cleaner before every weld
- Wrong filler metal: mismatched filler for the base metal alloy produces welds that look fine but fail mechanically or by corrosion
- Dissimilar metals without proper prep: welding carbon steel to stainless without the correct filler or technique produces HAZ cracking
- High-restraint joint design: joints that cannot move during cooling build up residual stress that causes cold cracking, especially in thicker sections
How to Fix a Bad Weld — 4 Scenarios
The right fix depends on the defect severity, the joint type, and what the weld is being used for. Here are the four options in order of increasing intervention.
Option 1: Grind and Re-Weld
The standard repair for most structural defects. Grind the bad section out completely with an angle grinder or die grinder until you reach clean, unaffected base metal — verified by dye penetrant if in doubt. Re-clean the surface, adjust your settings to address the root cause, and re-weld. This is the correct fix for porosity, undercutting (deep), cold lap, and crater cracks.
Option 2: Weld Over (Cosmetic Issues Only)
Acceptable only for purely visual defects on the surface of a sound, well-fused weld. A capping pass over a slightly rough surface bead is sometimes acceptable. Never weld over porosity, cracks, or cold lap — doing so traps the defect and makes it worse.
Option 3: Cut and Replace
Required when the defect is in a load-bearing fillet or a full-penetration groove weld on a structural member. Cut the entire weld out using a plasma cutter or air-arc gouging, prepare the joint properly, and re-weld from scratch with verified procedure and settings.
Option 4: Reject and Scrap
In safety-critical applications — pressure vessels, lifting equipment, aircraft components, structural connections in seismic zones — weld repair on certain joint types is not permitted by code. AWS D1.1, ASME Section IX, and similar codes specify which repairs are permissible. When in doubt in a certified environment, reject the part and issue a non-conformance report.
What Bad Welds Actually Cost
For engineers and procurement managers: the cost of a bad weld is not the cost of the rework. It’s the cost of when it’s found. The later a defect is discovered, the more it costs — by orders of magnitude.
| Defect Found At | Typical Cost Impact | Action Required |
| In-process (same shift) | Low — grind and re-weld, minimal delay | Rework immediately, re-inspect |
| Final inspection (end of production) | Medium — full rework + re-test cycle cost | Scrap or rework, delay delivery |
| At customer goods-in | High — return shipping, expedite cost, PO penalty | Formal 8D corrective action required |
| In service (field failure) | Critical — recall, liability, brand damage | Root cause investigation, field repair |
This is why weld quality programs and certified welding procedures exist — not as bureaucracy, but as cost control. A single field failure from a bad weld on a crane, pressure vessel, or structural connection can cost more than an entire year of production.
Frequently Asked Questions
What does a bad weld look like?
Look for irregular bead width, surface porosity (pinholes), visible cracks, heavy spatter around the joint, undercut grooves at the toes, and dark discoloration indicating overheating.
Can you weld over a bad weld?
Only for purely cosmetic issues. Structural or load-bearing bad welds must be ground out and re-welded from scratch. Welding over porosity, cracks, or incomplete fusion traps the defect and makes it worse.
How do you know if a weld is strong enough?
Visual inspection catches surface defects. For structural welds, a fillet break test or bend test confirms fusion. Critical applications require NDT — ultrasonic or dye penetrant testing.
What is the most common welding defect?
Porosity is the most common defect across all processes. It’s caused by contamination — oil, moisture, mill scale, or inadequate shielding gas coverage on the base metal or filler.
Is spatter always a sign of a bad weld?
Heavy spatter usually signals the wrong voltage/amperage setting or contaminated wire. Light, minimal spatter is normal in MIG welding. TIG welding should produce no spatter at all.
Can a bad weld be fixed?
Yes — most defects can be repaired by grinding out the bad section and re-welding with correct settings and prep. Critical structural welds in safety-critical parts sometimes require full joint replacement.
Summary: Good Weld vs Bad Weld
A good weld is uniform, fully fused, free from defects, and built to the correct profile for its application. A bad weld reveals itself through visual symptoms — irregular bead shape, porosity, undercut, cracks, heavy spatter, or cold lap — each of which traces back to a specific root cause in operator technique, machine settings, or material preparation.
The eight defect types covered here — porosity, undercutting, incomplete fusion, cracks, spatter, burn-through, distortion, and overlap — cover the vast majority of real-world weld rejections. Each one is fixable. Each one is preventable.
The most important takeaway: severity matters. Not all bad welds are equal. A cosmetic spatter issue and a cold lap crack in a structural joint are both defects — but one is cleaned up with a wire brush and the other shuts down the project until it’s cut out and re-welded.
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