Complete Guide On How To Fix An Undercut Weld 

 December 12, 2022

By  Chris

Welding is one of the most effective and promising processes for joining metals. However, due to some mistakes and lack of knowledge, there are defects caused by welding. One of them is the undercut weld.

Although different defects occur during welding, undercut in welding is one of the most common and dangerous defects that welders sometimes face. First, you'll learn about undercut welds, their causes, and how to prevent and fix them.

What is Undercutting In Welding?

As explained earlier, undercut in welding is a welding defect that occurs when joining metals. It is a groove formed during the welding process at the base of the weld.

Undercuts form due to high weaving, speed, more current, electrode size, arc length, inappropriate filler material, e.t.c. Due to that, the strength of welding decreases.

Types of Undercut In Welding

There are two types of undercut welding as follows:

Internal Undercut

Internal undercuts occur at the root of the welds on the base metal. They are usually seen as some small depressions, always on the sidewall of weld metals.

External Undercut

As the name suggests, external undercuts happen at the crown part or the top of the weld on the base metal. External welds are mostly at the edge of the weld metal, precisely on the base metal.

Causes of Undercutting In Welding

So, what brings these little villains to the stage? Below is a breakdown of reasons why welders experience undercut welds.

  • High amperage or voltage

  • Travel speed

  • Choice of Filler Metal

  • Imperfect Weaving Skills

  • Dirty surface

  • Wrong size of the electrode

Let's discuss them in detail:

High Amperage or Voltage

When a high current is allowed to pass through the entire system, the heat produced through the electrode will also be increased. Although there are some differences in the welding methods for MIG and TIG welding, high amperage or voltage is a significant cause of undercut welding.

When an electrode that is too hot due to high voltage touches the base metal, it creates grooves. Even though the filler metal is being added, grooves can be a significant problem when the electrode is too hot for the base metal.

This can occur in many ways. In some cases, the melted base metal can drain into the welded area and create unfilled spaces. Another way undercuts are formed is when the arc is too hot, making large beads.

Undercut Weld On Hand Rail

Travel speed

Another cause of undercut welding is the speed of the arc. The travel speed of the arc could lead to undercutting. It would be best if you kept the arc speed at an optimal range to create an aesthetic and spatter-free weld.

When the travel speed is too fast, there is not enough time for the welded metal to join correctly to the base of the metal. A welding arc that is too slow can also result in nasty penetration. Inversely, this is also a way to prevent undercut welding.

Choice of Filler Metal

The choice of filler metal can affect the temperature of the base metal. If the filler metal is not prepared for the kind of base metal and weld you will be working with, there will be some inconsistency in the temperature, which will cause a huge problem.

In addition, the choice of the filler metal can lead to uneven melting of the weld, which will cause undercuts on the base of the metal.

Imperfect Weaving Skills

During welding, weaving is the side-to-side movement with which the welding process is done. If you, as a welder, are not skilled in weaving, the process of welding can be daunting, and you will end up with an uneven weld.

An uneven weld, in this case, implies that there are unfilled areas in the joint—undercuts.

Dirty Surface

The metal surface must be dirt-free to get a clean and satisfactory weld. When the metal surface becomes dirty, this will result in undercuts. Like gas welding, having a smooth surface area free from dirt or debris makes welding easy.

Wrong Size of Electrode

Having the wrong size of the electrode can cause undercuts during welding. A massive electrode, for instance, will melt the base metal due to limited space, which causes the arc to touch the base metal.

How about having an electrode that is smaller than what's required? Well, in that case, the space to be filled won't be enough. Both large and small electrode situations can cause severe disturbance when welding.

Undercut Welding

How Much of Undercut In Welding is Acceptable?

Observations and studies in the welding society have established that undercuts may be acceptable, except they are more than 1/32 inch in depth. To be reasonable, undercuts must be below 0.55mm in length.

How to Prevent Undercut in Welding

Undercuts in welding can be significantly prevented when you pay a little extra attention to details. When you go through this guide section, you will be saving yourself stress from having to go over a weld that has already ended. You can kiss undercuts goodbye by adhering to the steps here.

Here's a comprehensive list of measures to prevent undercuts in your welds.

Ensure you have an adequate arc gap

When your weld lacks an apt arc gap, it is prone to undercuts. One way to prevent this as a welder is to keep an eye on the arc length. Always ensure that you have the correct size before proceeding with your welding. This will reduce the spatter and, ultimately, the undercuts.

An ideal way of determining which arc gap/length to use for your weld is to ensure that it does not exceed your electrode's diameter.

Have Good Weaving Skills

A good weaving skill always comes in handy when avoiding undercuts as much as possible in your weld. You do this by making sure that you maintain tight spaces between each weave. This indeed comes as a result of the expertise and experience of the welder.

In addition to that, you also need to maintain optimal speed when weaving. It would help if you kept a fast pace as you put more effort into holding the sides so as not to affect the integrity of your weld. Weaving this way will avoid undercuts.

Use a Good Shielding Gas

As a professional practice, the type of shielding gas you'll be using should depend on the material of the electrode as well as the metals you will be working with. For instance, in TIG, Argon as a shielding gas can only be used as an electrode of tungsten material. This will diminish spatter and ultimately avoid undercuts in your weld.

Similarly, you must ensure that the metal you work with is compatible with the electrode material to give you nice welds. Proper shielding gas prevents poor weld puddle contamination.

Avoid Too High or Low Amperage

If you want to avoid undercuts in your welding project, you must pay attention to high amperage as much as you're concerned about low amperage. Either of these situations can make welding tricky and cause undercuts. Working with an optimal current in combination with optimal arc length and gun speed gives you a sound output free of undercuts. Once you combine these together, you can get your desired results without hitches.

Get Your Angles Right

Another preventive measure to undercut your welding project is to ensure that the gun is positioned at the right angle required for your weld. You can also research the right angle to maintain for your welding task.

Here's a pro tip; always position your welding gun the same way you push the weld. In the same way, for different welding types, you will need to find the appropriate angle. For example, if welding wire, you must maintain the gun at 10 to 15 degrees.

Smooth Work Surface

Welding is a process that demands concentration and steady hands. When your work environment—the workbench surface- isn't smooth, you may have difficulty avoiding undercuts. When you notice your workbench or the metal surface isn't smooth enough, try fixing that before proceeding with your work.

Allow the Base Metal to Cool

Undercuts can easily be caused when you do not exercise patience for the base metal to cool as you weld. A simple trick is to segment your welding project and allow the metal to cool before proceeding to the next segment.

Have Eyes on What You Do

What should stand out in your undercut preventive measures is visibility. This starts with getting the right welding gear for your project. More important than getting the right angle for the project or ensuring a balanced workbench is the ability to keep an eye on what you do.

In addition to having welding gear that allows you to see your work better, a welding glove that gives you a better grip will also come in handy. Lastly, ensure that you are positioned right where you can see all the action when the metal and the arc meet.

Metal Surfaces Should Be Free From Dirt

Stains and debris can cause undercuts in your welds. To avoid spatter, always clean the metal's surface to ensure it is free of particles. For welding processes that involve aluminum and stainless steel, make sure to rub off any form of oxidation on the surface of the metal.

What is metal fabrication?

How to Fix Undercut In Welding

Although they result from flaws in your welding skill and technique, it is crucial to learn how perfect your weld puddle. Fixing undercuts usually takes more effort. But when cuts cannot be avoided, you can take measures to make them disappear.

Below are four methods to fix undercut in your welds:

Dab and Move Method

The dab and move method is relatively easy and takes less time to complete. As the name suggests, you must keep dabbing, even as you work on your previous weld.

You can start by placing the electrode in the middle of the low toe as you move forward. As you move forward, you split and dab the weld bead to create enough cover on the undercut.

Weave Method

The weaving method is the most common method used for pipe welding. Professional welders use the weave method as an effective way to cover up undercuts in their welding. Like the dab and move method, the weave method consumes less time and is more intricate.

In the weave method, the electrode goes back and forth in a quick horizontal manner. All you need is a knack for weaving at the appropriate speed so that you don't have to worry about welding as you proceed.

Stringer Bead Method

The stringer bead technique is easy to learn and is used explicitly for metals with a certain level of thickness. This method is recommended as an effective way to cover undercuts as it gives better coverage over the welded area.

Unlike the weave method, the stringer bead method takes time to complete as you have to make different welds. This will give you better-looking welds with well-covered undercuts.

Sea Crescent Method

In the sea crescent method of fixing undercuts, you move the electrode in a way that mimics the sea crescent shape. You start by making the weld pool, and then you begin to lay the wire over it.

Moving the electrode in a motion similar to the sea crescent shape is an excellent way to help you give a smooth flow as you attempt to cover parts that need to be covered to get a cleaner look.


Undercut in welding constitutes a significant problem that can be avoided easily, yet many welders fail to prevent this due to a lack of keen eyes or good skills. While welding, it is vital to prevent undercut welding by examining your welding method and polishing it until it shines bright.

Always keep an eye on the amperage when working on unique materials. Do not forget to pay attention to details like the arc length, the material and size of the electrode, the surface of your workbench, e.t.c.

Welding can be more fun and beautiful when you avoid splatters as much as possible. Should you have undercuts in your welds, do not forget to deploy the above methods in dealing with them.

For more details about welding, explore our welding page today.


Average guy that likes to build things and teach others what I learn. Family comes first. Steel, Jeeps and off-roading are all fighting for second place.

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