Rocker Panel Repair on Galvanized Steel Near a Fuel Tank
A 2010 van needs its lower rocker panel replaced after rust perforation. The original panel is 1.2 mm galvanized steel, spot-welded along the top flange and seam-welded at the front and rear. The technician proposes to cut out the rust, overlap a 1.5 mm mild steel patch by 20 mm, and run a continuous MIG bead along the overlap. He will then grind the weld, fill any pits with body filler and paint. The shop has a 180 A MIG welder with 0.8 mm solid wire and CO2, but no spot welder, no weld-through primer and no cavity wax. The fuel tank is 300 mm from the repair area and is half full. The van is due for roadworthiness inspection in two days. The workshop has a fire blanket, a CO2 extinguisher and a fume extractor, but the extractor is not positioned at the sill. The technician has not welded galvanized steel before.
1. Assess the proposed repair. What is wrong with the joint, material and safety plan?
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The proposal is not acceptable. A continuous MIG bead on a 20 mm overlap will concentrate heat, warp the 1.2 mm panel and leave a crevice between the sheets where moisture will start rust again. Grinding the weld and filling pits with body filler puts filler over a welded joint; it will crack. A 1.5 mm patch is thicker than the parent panel and changes stiffness. Welding galvanized steel produces zinc fume and porous welds unless the coating is removed from the weld zone and extraction is used. The half-full fuel tank 300 mm away is a serious fire and explosion risk; it must be removed or the area made safe before any welding. The unused fume extractor is also a hazard.
2. Specify the repair method and sequence you would use, including distortion control and corrosion protection.
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Remove the fuel tank or have it drained and purged, and fit a fire blanket and fire watch. Drill out the factory spot welds and remove the rusted rocker, cutting back to sound steel. Clean the zinc coating 10 to 15 mm from the weld zone, apply weld-through primer to mating flanges, and fit a new panel of the original thickness. Plug weld the top flange at the factory spot-weld positions; use a butt joint with a backing plate or a flanged joint at the ends, and stitch weld where a seam is original. Use 0.8 mm wire, set the welder for thin sheet, tack first and alternate stitches, allowing each to cool. Planish lightly to control distortion. Do not run a continuous bead along an overlap.
3. What inspection and final-protection checks must be made before the van goes for roadworthiness inspection?
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After welding, clean and neutralise the zinc fume residue, apply zinc-rich primer to welds, seam sealer to joints, and cavity wax inside the rocker. Check plug-weld penetration and diameter, look for burn-through and gaps, and confirm the panel is flush with the door and sill. Refit the fuel tank and check for leaks and vapour before starting the engine. Record the repair and ensure the fume extractor is positioned at the sill. If the welds are porous or the panel distorts, cut out and redo rather than filling.