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To choose the right joint for the right material
When a damaged panel is to be replaced, the new part must be fastened so the joint becomes at least as strong as the factory's. The choice of joining method is not free: it's controlled by the material, by its placement in the structure, and by the manufacturer's repair instructions. An incorrect joint in a load-bearing part can weaken the entire vehicle.
The factory assembles a large part of the body with spot welding, where two electrodes press the plates together and send current through so the metal melts point by point. The method is fast, provides controlled heat and is used on overlapping plates. During repair, one often mimics the factory's spot welds with a spot welder where it is possible to access from both sides.
MIG/MAG welding feeds material with a wire electrode under a protective gas and is used where access is only possible from one side or where a seam needs to be filled. Heat must be controlled carefully: too much heat deforms the plate and can change the steel's properties. Therefore, short passes are often welded with pauses so heat does not spread unnecessarily.
Many modern cars are partially assembled with structural adhesive often in combination with rivets — especially where different materials such as steel and aluminium meet or where heat would damage the sheet or paint. The adhesive distributes the force over a larger area and seals the joint at the same time while the rivets hold the parts in place while the adhesive cures. Always follow the manufacturer's instructions for pre-treatment and curing time.
A good joint is judged on both appearance and strength: an even weld bead without pores correct number and placement of points and a bonded joint that is filled and without air pockets. Follow the manufacturer's requirements for where and how there may be joints — it is here that the difference between a safe and a dangerous repair lies.