Straight answers to the questions people ask most when welding and preparing vehicle repairs.
Which MIG wire and gas suit car bodywork and chassis repairs?
General-purpose mild steel MIG wire is widely used for vehicle repairs. Thinner wire is normally chosen for thin body panels and thicker wire for heavier sections, so match the wire diameter to the metal thickness and to what your machine and torch are set up for.
Shielding gas is usually an argon and carbon dioxide mix, which gives a smoother arc with less spatter, while pure carbon dioxide gives deeper penetration but more spatter. Check the data plate and manual for your own machine for the wire sizes, gas and drive rolls it is designed to use, and follow the vehicle maker's repair information.
How should I set up a MIG welder for panel work?
Start with a low voltage and a wire speed to match, using the maker's chart as a guide, and test on offcuts of the same thickness. Keep the wire stick-out short, the earth clamp on bare, clean metal close to the weld and the torch angled steadily.
A good weld has a steady, even crackle, with no popping or hissing. Wire that stubs into the work means the voltage is too low or the wire too fast, and a big burning arc with spatter means the reverse. Adjust one setting at a time.
How do I stop thin panels warping or burning through?
Use the lowest settings that will fuse the joint, and weld in short stitches or tacks rather than a long continuous run. Skip around the repair so heat does not build up in one place, and let the metal cool between welds.
Good fit-up helps, as a big gap invites burn-through. A copper backing strip behind the joint can absorb heat, and heat-absorbing paste or a damp cloth next to the weld can help. Dress welds gently, because over-grinding thins the panel.
What is a plug weld and when should I use one?
A plug weld replaces the original spot welds where one panel overlaps another. You drill or punch a small hole in the outer panel, clamp the panels tight together and fill the hole with weld, which fuses the two layers.
It is a common way to join lapped panels such as sills, floor sections and wheel arches when you do not have a spot welder. Practise on offcuts first, keep the panels in close contact, and weld in short bursts so that heat does not build up.
Can I weld structural and safety-critical parts of a vehicle?
Chassis rails, suspension mounts, seatbelt anchorages and sills are safety-critical. They must be repaired to the vehicle maker's repair guidance, using the right materials and methods, and a poor repair could be dangerous and may not pass an MOT inspection.
If you are not trained and experienced in structural repair, or are unsure what is allowed, have the work done or checked by a qualified professional. Never guess on anything that carries load or protects the occupants.
What should I do after welding to stop the repair rusting again?
Clean and dress the weld, then protect the bare metal straight away. Treat the area with a suitable primer, seam sealer where seams need sealing, underseal where appropriate and cavity wax inside closed sections, and finish with paint to match the vehicle.
Check the vehicle maker's repair information and the coating supplier's instructions before using any weld-through primer between panels, as it is not suitable for every joint or welding method. Bare steel rusts quickly, so do not leave the repair unprotected for long.