Generally, you cannot effectively spot weld over paint. While spot welding is a fast and common method for joining sheet metal, paint acts as an electrical insulator and a barrier, preventing the direct metal-to-metal contact necessary for a strong, reliable spot weld.
Why Spot Welding Over Paint Is Difficult
Spot welding works by passing a high electrical current through two or more layers of metal held together under pressure. The resistance to the current at the interface of the metal sheets generates heat, melting a small spot that fuses the layers together when it cools.
Paint interferes with this process in several ways:
- Electrical Insulation: Paint prevents the high current needed for welding from flowing effectively between the metal surfaces.
- Poor Contact: The paint layer creates a gap or inconsistent contact area, reducing pressure effectiveness and current flow.
- Contamination: The paint material can burn, vaporize, or become trapped in the weld zone, leading to porosity, reduced strength, and inconsistent weld quality. It can also foul welding electrodes.
Attempting to spot weld through paint often results in poor welds, surface burning, excessive expulsion (splatter), and damage to the welding electrodes.
Welding Processes That Can Handle Coatings
While spot welding typically requires clean surfaces, some other welding processes are designed to handle surface coatings like paint or rust. These processes often utilize methods that overcome the barrier presented by the coating.
Stud Welding Through Paint
One example of a process capable of welding through paint is stud welding. This method uses an electric arc to melt the end of a stud and a portion of the base metal, creating a molten pool into which the stud is plunged.
As referenced, the key to stud welding through paint lies in the extreme heat of the arc:
- The arc in a stud weld reaches temperatures between 6,000-7,000°C (10,800-12,600°F).
- This intense heat immediately vaporizes the small amount of paint directly around the point of penetration.
- This vaporization clears the way, allowing the weld stud to plunge into the molten pool of base metal and form a good quality weld.
- Therefore, it is possible to stud weld through paint because the process eliminates the coating during the weld cycle.
In contrast to stud welding's arc vaporization method, spot welding's resistance heating principle is unable to reliably burn away or displace the paint layer sufficiently to create a sound weld.
Conclusion
In summary, while processes like stud welding can accommodate paint due to their high-temperature arc vaporizing the coating, spot welding generally requires the paint to be removed from the welding area to ensure proper electrical contact, current flow, and ultimately, a strong and reliable weld.
