Corrosion Protection After Rear Rail Sectioning
2020 minivan rear rail sectioned at the OEM location and MIG-welded with 0.8-mm wire. The e-coat is burned back about 30 mm on each side of the weld, seam sealer was removed over a 300-mm length, and the interior cavity is accessible only through a 25-mm hole. The technician plans to spray rubberized undercoating directly over the weld and skip cavity wax because the vehicle is registered in a dry climate. The repair also includes a new rear crossmember with plug welds. The shop has zinc-rich primer, seam sealer, and cavity wax with a flexible wand. The weld has been ground smooth.
1. What is wrong with the proposed corrosion protection, and what sequence should be used?
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The undercoating-over-weld plan is incomplete. Clean the weld area, remove loose sealer and undercoating, and apply zinc-rich primer to all bare steel and burned e-coat within the repair area. Allow it to cure. Apply seam sealer to lap joints and plug-weld seams after priming. Then apply cavity wax inside the rail through the 25-mm access hole using a flexible wand, and apply undercoating only over primed surfaces. Dry climate does not eliminate corrosion risk from road splash and condensation.
2. How should the technician protect the interior cavity and the crossmember plug welds?
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Use the 25-mm hole with a wand that reaches both directions; verify coverage with a borescope if available. Cavity wax should be applied after paint and seam sealer, not before welding. Avoid blocking drain holes. For the crossmember plug welds, prime the mating surfaces before welding if the OEM allows weld-through primer, then prime the finished welds. Do not trap moisture between sealer and bare metal.
3. What documentation and final inspection should be completed?
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Document the corrosion protection materials, lot numbers, and coverage. Check that seam sealer matches OEM texture and does not bridge drain paths. Verify e-coat repair around the weld and under the crossmember. A final inspection should include the interior cavity and exterior underbody. If the cavity wax cannot reach the upper rail, add an access hole only where the OEM permits it. Record that the dry-climate argument was rejected and that cavity protection was applied.