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Undercut is a groove melted into the base metal alongside the weld toe that the filler metal does not completely fill. It reduces the effective thickness of the joint and creates a sharp notch where cracks can start. In stick welding, undercut is usually caused by excessive heat, an arc that is too long, or travel that is too fast. The good news is that it is usually a technique and setup problem, not a mysterious defect.
Identify Undercut Before It Causes Trouble
After the weld cools, inspect both toes of the bead under bright light. Undercut appears as a continuous or intermittent channel beside the weld rather than a smooth transition into the parent metal. Run a gloved fingertip across the joint, but do not rely on touch alone; a shallow groove can still be visible and significant.
For critical work, use a weld fillet gauge or bridge cam gauge to measure the groove. The permitted depth depends on the welding code, joint type, and application. A decorative repair may tolerate a minor mark, while structural or pressure-related work may not. If the groove is clearly visible, catches a wire brush, or runs for a substantial distance, treat it as a defect rather than painting over it.
Set the Machine for the Electrode
Start with the amperage range printed on the electrode package. As a practical starting point, a 1/8-inch 6010 or 6011 electrode often runs around 75–125 amps, while a 1/8-inch 7018 commonly runs around 90–130 amps. The correct setting depends on position, joint thickness, polarity, and the machine, so use the low-middle part of the range first and adjust after observing the bead.
| Symptom | Likely cause | First correction |
|---|---|---|
| Groove along one or both toes | Too much amperage, excessive arc length, or excessive speed | Lower amperage 5–10 amps, shorten the arc, or slow slightly |
| High, narrow bead with poor tie-in | Traveling too fast or using too little heat | Slow down slightly or raise amperage in small steps |
| Wide, flat bead with excessive spatter | Too much current or an unstable long arc | Reduce current and keep the arc length consistent |
| Undercut on only one side | Incorrect work angle or uneven hand movement | Center the electrode and pause briefly at that toe |
Do not solve undercut by making a large jump in amperage. A change of 5 to 10 amps is easier to evaluate and is less likely to create burn-through, excessive penetration, or a bead that becomes too wide. Make sure the machine is set to the polarity recommended for the electrode. Incorrect polarity can produce a harsh arc, poor penetration, and erratic bead shape.
Control Arc Length and Electrode Angle
Arc length is one of the most common causes of undercut. A long arc spreads heat across the surface, increases spatter, and makes the arc dig into the edges without depositing enough metal at the toes. With most stick electrodes, keep the arc roughly equal to the diameter of the bare electrode core. For a 1/8-inch rod, that means an arc close to 1/8 inch. A 7018 generally prefers a short, quiet arc; a 6010 may need a slightly more active whip-and-pause motion, but it still should not be held far away.
Use a travel angle of about 10 to 15 degrees in the direction of travel for many flat and horizontal welds. Keep the work angle centered over the joint. If the electrode points too far toward one plate, that side receives more arc force and heat, increasing the chance of undercut on the opposite toe.
For a fillet weld, aim the arc into the root and direct enough heat toward each sidewall to fuse both plates. Do not simply drag the electrode along one edge. In vertical welding, use a controlled weave or small triangles only when the electrode and position call for it. Excessive weaving widens the molten puddle and makes the toes harder to control.
Adjust Travel Speed and Puddle Control
Moving too fast leaves a narrow, convex bead and gives the molten metal no time to wash into the toes. Moving too slowly can also cause problems, including a wide, overheated puddle and slag entrapment, so the answer is not always “go slower.” Watch the puddle, not just the electrode tip. The bead should fill the joint evenly, with the edges briefly washing into the base metal without digging a visible trench.
If undercut appears at the edges, pause for a fraction of a second at each toe while maintaining a short arc, then move across the center. With a small weave, keep the weave width modest—often no more than two to three electrode diameters—and avoid pausing at the center. On a straight bead, a slight reduction in speed may be enough.
Practice on the same thickness and joint configuration as the real job. A simple coupon made from similar steel is cheaper than grinding out a defective weld, especially when learning a new electrode or welding position.
Prepare the Joint and Electrodes
Remove mill scale, rust, paint, oil, and moisture from the weld area. Contamination can make the arc unstable and hide poor fusion beneath spatter. A clean steel wire brush is useful for light scale; use a grinder when the surface is heavily rusted or coated. Wear proper eye and respiratory protection when grinding painted or galvanized material.
Keep low-hydrogen electrodes such as 7018 dry according to the package instructions. Damp rods can produce porosity and hydrogen-related cracking. A sealed package is often sufficient for occasional home repairs, while a rod oven is worthwhile for repeated structural work. For basic noncritical repairs, buying a small pack of fresh electrodes is usually cheaper and simpler than maintaining storage equipment.
A stable [welding work clamp](https://www.amazon.com/s?k=welding+ground+clamp+heavy+duty&tag=powertoolsdiy04-20) and clean contact point also help keep the arc consistent. Place the clamp on bright, clean metal rather than over paint or rust.
Repair Existing Undercut
Do not bury deep undercut under another pass without preparation. Grind the defect to remove the sharp notch and feather the surrounding metal, then clean the area thoroughly. For a shallow groove, a small corrective pass may be enough. Use the same current-control principles: short arc, steady travel, and a deliberate pause at the affected toe.
If grinding would remove too much section, if cracks are present, or if the weld is part of a load-bearing or code-regulated assembly, stop and follow the applicable repair procedure. A qualified inspector or welding professional may be required. For routine shop work, a [fillet weld gauge](https://www.amazon.com/s?k=fillet+weld+gauge+set&tag=powertoolsdiy04-20) makes it easier to verify that the repaired profile is smooth and adequately filled.
Use Gear That Improves Control
Equipment cannot replace technique, but poor visibility makes undercut more likely. Use a properly fitted auto-darkening helmet with a clear lens, suitable shade, and a clean cover lens. A basic helmet is fine if it switches reliably and lets you see the puddle. Features such as adjustable sensitivity and delay are useful when moving between welding and grinding, but they are not essential for preventing undercut.
Choose gloves that protect your hands without making the electrode holder difficult to control. Thick gloves are useful for heavy work, but a bulky fit can encourage an unstable grip. Keep both hands supported when possible, brace your forearm, and position the work so you are not welding at full reach.