Why coastal cars rust differently
Salt in the air settles on and in every exposed metal surface, and unlike inland dust, it actively accelerates corrosion rather than just sitting there. A car that's spent its life in Durban, Cape Town, or a smaller coastal town has been exposed to this constantly, regardless of how well it's been washed — rust starts from the inside of panels and cavities as often as from visible surface damage, which is exactly why it's easy to miss on a quick walk-around.
Just as the site's fuel price pages distinguish inland from coastal pricing, the same inland/coastal split applies here: a coastal car generally carries more corrosion risk per year of age than an equivalent inland car, all else being equal. It's not a reason to avoid coastal cars — it's a reason to inspect them more thoroughly.
Where to check, in order of priority
| Area | Why it rusts first |
|---|---|
| Wheel arches (front and rear) | Constant exposure to road spray carrying salt, and hardest for a car wash to actually reach |
| Sills and rocker panels | Trap moisture underneath the vehicle where it can't dry out |
| Boot floor and spare wheel well | A low point that collects moisture and is rarely inspected by owners |
| Subframe and suspension mounting points | Structural — rust here affects safety, not just cosmetics |
| Brake lines and fuel lines | Safety-critical — a corroded brake line can fail under pressure |
| Door bottoms | Drain holes designed to let trapped moisture out often become blocked with debris over time |
Two cheap tools that reveal what's hidden
- The magnet test — a small fridge magnet won't stick to filler or fibreglass, but it will stick to bare or painted steel. Run one gently over a suspicious panel; if it won't stick in a spot where the surrounding metal holds it firmly, there's non-metal filler underneath, almost always covering a repaired rust hole or dent.
- The knuckle tap test — tap gently along a sill or wheel arch with your knuckle. A solid panel gives a consistent, slightly dull thud. A hollow or crumbly-sounding response, or an area that flexes slightly under light pressure, suggests the metal has thinned or rusted through beneath the paint.
Check the drain holes
Doors and sills have small drainage holes designed specifically to let moisture that gets in (rain, condensation, a wash) drain back out rather than sit and rust the panel from the inside. Over years these holes can clog with dirt and debris, trapping moisture exactly where it does the most damage. Check that they're clear where you can access them — a simple, low-cost thing to verify that most owners have never thought about.
A paint thickness gauge finds filler you can't see
Combine the magnet and knuckle tests with a paint thickness gauge reading across suspicious panels — a reading noticeably higher than the surrounding area confirms filler, which combined with a hollow knuckle-tap sound all but confirms a rust repair rather than a simple respray. See our paint thickness gauge guide for how to use one properly.
On an older, lower-value car, structural rust repair — cutting out and welding in new sill or subframe sections — can cost more than the car is actually worth. Treat significant structural rust as a walk-away trigger on a budget car, not just a negotiating point.
Corrosion anywhere near brake lines or fuel lines is a safety issue, not a cosmetic one. Have this assessed by a mechanic immediately rather than proceeding on your own judgement.
Check the six spots, in order, every time
Wheel arches, sills, boot floor, subframe, brake and fuel lines, and door drain holes — checking these six areas specifically, with a magnet and a knuckle tap, catches the large majority of coastal rust before it becomes your problem instead of the seller's.
Frequently asked questions
Get the full 60-point inspection checklist — free
The complete inspection system for SA used car buyers. Print it. Take it to every viewing.
Rust, chassis integrity, exhaust, CV joints, suspension and brakes — the Underbody Inspection Guide covers the checks most buyers skip because they're out of sight.
Get the Underbody Inspection Guide — R75 →