Rust repair: what can be saved, and what a proper fix involves

Surface rust on a panel can be sanded back, treated and painted. Rust that's gone through the metal has to be cut out and new steel welded in. Anything in between is a judgement call, and the honest answer depends on where it is and what the car's worth. Queensland's humidity, salt air near the coast and summer storms mean we see plenty of it at Moss St.

Why cars rust here

Moisture gets into stone chips, scratches, seams and drain holes and sits. Salt (from the coast or a boat ramp) speeds it up. Heat and humidity keep it going. Older cars with less factory corrosion protection, and cars that have had a cheap repair where the primer was skipped, go first.

Surface rust versus rot

Surface rust is on the outside of the metal: a chip or scratch that's gone orange, a bubble under the paint, a rusty bonnet edge. The metal underneath is still solid. This is repairable: sand to bright metal, treat with a rust converter or epoxy primer, fill if there's pitting, prime and paint.

Scale is rust that's started to flake and pit the metal. Still repairable if the metal is thick enough after cleaning.

Rot is rust that's gone through. You'll see holes, or the metal crumbles when pressed. The only proper fix is to cut the rotten section out and weld in new metal, then seal, prime and paint. Filling a hole with fibreglass or filler is a bodge that comes back within a year.

Where to look

• Lower doors and door bottoms (drain holes block)
• Sills (rocker panels) and jacking points
• Wheel-arch lips, especially rear
• Bonnet and boot edges
• Around the windscreen and rear window seals
• Boot floor and spare-wheel well
• Under the tailgate on wagons and utes
• Ute tubs, especially the front wall and tie-down points

Bubbling paint is the usual first sign. By the time it bubbles, the rust has been there a while.

What a proper repair involves

1. Strip the paint back to see the real extent. It's always bigger than the bubble.
2. Decide: treat and paint, or cut and weld.
3. For rot: cut back to clean metal, fabricate or buy a repair section, weld it in, dress the welds.
4. Treat the back of the repair as well as the front, with cavity wax or seam sealer, so it doesn't rust from behind.
5. Epoxy primer, filler if needed, primer, block sand.
6. Colour and clear in the booth, blended into the panel. See spray painting.

The step most cheap repairs skip is number 4. Rust that's painted over from the front comes back from behind.

Is it worth it?

On a car you want to keep, almost always. On a car you're selling, a bonnet edge or a wheel-arch lip is worth doing because buyers walk away from rust. Rotten sills or chassis rails on an old car can cost more than the car, and they'll fail a safety certificate if they're structural. We'll tell you honestly which side of the line you're on.

Stopping it coming back

Wash the car regularly, including under the wheel arches. Clear the drain holes in the doors and sills. Touch up stone chips before they rust. Keep it out of the salt air when you can.

Send photos of the rust (close-up and from a step back) through the quote form and we'll tell you what it needs. For big areas we'll want to see the car. More on the panel beating page.