How to Tell a Good Weld From a Bad One

Whether you are checking work you paid for or sizing up a used trailer before you buy it, you can learn to spot a bad weld in about two minutes. You will not catch everything a pro would, but the obvious problems are obvious once you know the tells.

What a good weld looks like

• Consistent and uniform. The bead is even in width and height the whole way, with regular ripples, like a stack of dimes. Sudden changes in size mean the heat or hand was all over the place.
• Tied into both pieces. A good weld melts into both parts and blends in smoothly at the edges. It should look like it grew out of the metal, not like it was laid on top.
• The right size. A weld that is too small for the joint is weak no matter how neat it looks.
• No gaps or holes. Solid and continuous, start to finish.

The red flags

• Cracks. Any crack, anywhere, is a failed weld. No exceptions. (More on why that happens in Why Welds Crack.)
• Porosity. Little pinholes or bubbles on the surface mean gas got trapped in the weld, like bubbles in a bad pour of concrete. Weak and a sign of dirty metal or bad technique.
• Undercut. A little groove melted into the base metal right along the edge of the weld. It thins out the parent metal and creates a stress line, a common hidden weakener.
• Sitting on top (cold weld). If the bead looks like a worm laid on the surface with a sharp edge where it meets the metal, it probably never fused. Pretty but useless.
• Spatter everywhere. Little balls of metal sprayed around the weld will not fail the joint by themselves, but they tell you the settings or the hand were sloppy.

How the pros go further

For anything structural or safety-critical, looks are not enough. Welders and inspectors use dye penetrant (a liquid that seeps into and reveals surface cracks you cannot see), and for the serious stuff, ultrasonic or X-ray testing that checks inside the weld. You will not need that for a railing, but it is why real structural work costs what it does.

Here is the rule of thumb: a pretty weld is not always strong, but an ugly weld is almost never strong. If something you have looks off, we fix a lot of other people's welds, and we will tell you straight whether it is sound.