If you’re buying a used vehicle, it’s essential to know how to identify corrosion on a used car. Rust reduces safety and value, and it can progress quickly if it’s been covered up by poor repairs.

Critical zones: where rust attacks first

Do your inspection in daylight or under strong lighting, with clean surfaces.

  • Fender lips and wheel arches (inner arches, especially behind liners and where dirt collects)
  • Sills/rocker panels and the floorpan, especially around jacking points and panel seams
  • Suspension mounts, subframes, and attachment areas (around bolts and seams)
  • Lower edges of doors and lids (trunk and hood), under seals and trim strips
  • Windshield and rear-glass frames, roof channels, and around drain outlets
  • Fuel-filler neck and the area around the cap
  • Heat shields and exhaust hangers Tip: lift door seals with your fingers and look underneath. Use a flashlight to inspect inner edges and seams.

How to spot repair signs and hidden corrosion

Rust is often concealed under paint or underbody coating. Watch for:

  • Bubbles and micro-cracks in the paint, especially along panel edges and seams
  • “Orange peel” and uneven paint texture on the same panel
  • Overspray on rubber, plastic, or seals
  • Sharp paint edges or tape lines along panel borders (a masking sign)
  • Thick, freshly applied underbody coating only on one section of the car
  • Visible sanding marks, patch panels, uneven or rough welds
  • Bolts and clips with fresh tool marks or that look new on an older vehicle
  • Condensation or a damp smell inside; lift carpets and check for rusty specks on the floor

Simple pocket tests and tools

  • Small flashlight and mirror: look behind edges and into cavities
  • Magnet wrapped in a thin cloth: weak or no pull can indicate filler instead of metal
  • Gentle tapping with a plastic handle: a dull thud often reveals filler and hidden rust
  • Touch and look: roughness under your fingertips, bulges, or soft edges suggest rust under paint
  • Check drain holes: clogged drains trap water and accelerate corrosion Note: don’t damage protective coatings, and never get under the car without stable, secure support.

When it’s time to call a professional

Call a body repair specialist or independent expert when:

  • You see perforation (pinholes) or a crust of rust that crumbles at a touch
  • There are long lines of bubbles along seams, sills, or around suspension mounts
  • You notice signs of major bodywork of questionable quality (thick filler, rough welds)
  • You’re unsure how far damage extends under coatings or paint A professional inspection can reveal structural issues and help you judge whether repair—or the purchase—is worthwhile.

How to document findings and decide

  • Photograph each suspect spot up close and from a distance
  • Note the location (e.g., right sill, left rear arch) and description (bubbling, bulge, crack)
  • Compare left vs. right sides and the upper and lower edges of the same panels
  • Light surface rust can often be addressed in time; advanced or structural rust usually means bigger jobs and higher risk For more practical tips and topics related to choosing and inspecting used vehicles: Browse all guides.