Use A Fridge Magnet To Test Used Cars For Damage

After we posed the question yesterday, Lifehacker readers came through with plenty of handy tips for buying a car. One particularly useful notion? Take a fridge magnet along with you to check for rust and filled areas.

Reader StevoTheDevo points out that running a magnet over the body will identify areas that have been filled. And Tomas notes that it’s also a good rust detector:

Apply it in the typical spots that rust would form, if the magnet doesn’t stick, then rust’s been cut and bogged.

Especially in coastal areas, that seems like a sensible precaution.


  • ne point I’d like to make, is unless you’ve had a car parked next to a beach all its life, you’re pretty safe not to find rust in most modern cars that are less than 20+ years old. With the manufacturing processes that have been employed since the 70’s not a huge number of cars survive to this age, so its largely a redundant concern.

    Using a magnet to identify rust is a method thats been around for a while. Really though, its one of those things that sound more practical than they are.

    Bog is used to patch up small holes where the rust has been wire-brushed or grinded out, typically smaller than 0.5-1″. With a decent amount of steel surrounding such a hole, you’re still going to get a false positive on your magnet test.

    Secondly, another means of patching larger rust holes after its been cut out, is use of fibre-glass patches and resin. This maintains a lot of the strength of the panel, and isn’t by any means necessarily a bad fix (similarly to removing a tumour, the more “healthy” metal you remove surround rust, the less likely the rust is to return). Of course you’re likely to get a positive on a magnet test, but I’d much rather find fibreglass than bog, which has no structural strength at all.

    Finally, the best method of patching rust is to cut out the rust and surround metal, and weld a new sheet in. If done correctly by a reputable panel beater, the observer need never know rust was present in the first place, and a magnet is going to be of no use in identifying a patch up job or not.

    Of course though, all of the above fixes depend on the rust being on a non-structural panel, as opposed to a structural part of the chassis. No patching is really really safe on a monocoque chassis employed in almost any car built in the last 30-40 years, as it compromises the strength of the whole structure. Best advice that I can give if you’re concerned about rust, check all the metal in the engine bay, the A, B, C (and D if the car has them) pillars, and underneith the car (especially under the doors). Look for bubbling, or any twists, buckles or dents in the metal. Be especially carely to inspect around any bolts and welds in the chassis, and where any trim has been attached (moisture loves hiding in nooks and crannies).

    For surface rust, make sure all the panel’s look straight and even, with no flat spots on a curved panel etc. It also sounds obvious, but check to make sure the paint is the same across all panels, as its often difficult to create an exact colour match when a panel is resprayed following a rust repair.

    • I have a 5 year old Mazda3 that has significant rust starting on the inside of the doors and the rear quarter panels. Quite disappointing.

      Another problem with this method is the number of cars that use panels of a material other than steel. Of course, Corvettes have had plastic bodies for over 50 years as did the Fiero and most Saturns, but more and more cars have some panels made of either plastic or aluminum. The magnet isn’t going to stick anywhere on those parts.

  • I live in the rust belt – in other words we use salt on our roads in the winter – so rust is a big issue, we have been using this technique for years, but it doesn’t work on plastic cars when the frame rusts out, you still have to get down and look.

    My best advise is to take it to a shop that you trust and have it inspected, if it is a reputable seller they would not have an issue with this.

Show more comments

Log in to comment on this story!