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I realise that can't be answered without me saying how serious the corrosion is. The context is that I'm looking at a 2004 Ford Focus on eBay and the MOT history on the DVLA web site reports an advisory at the last MOT six months ago:

O/S REAR INNER SILL IS RUSTY

None of the previous MOTs reported any corrosion, just the usual headlight bulbs, brake pads worn, etc. The last MOT just states the above in the Advisory section without any more detail.

Aside from the corrosion problem the car looks exactly what I want. I suspect the car will sell cheaply as there doesn't seem to be much interest in it, so I don't mind paying for the sill to be fixed as long as:

  1. it doesn't cost a fortune

  2. it isn't the start of massive bodywork problems

So what I'm hoping you can tell me is whether sill corrosion is fairly routine on 13 year old Focuses and easily fixed, or whether it's an ominous sign and I'd be well advised to walk away.

I don't have the kit (or skill) to fix it myself, so I'd be paying my friendly mechanic to do the work.

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The sills are a vital structural part of a unibody chassis, they provide strength and rigidity which is important in the event of an accident, this is why it's been flagged up in the MOT.

Without a photo, its hard to tell how bad it is, but even with a photo, what you can see is normally the tip of the iceberg. Upload one and you might get more specific responses.

Repairing one patch of rust is normally where massive bodywork starts, then you'll need to repair another patch - probably on the other sill. Any repairs that are carried out will arguably be compromised anyway because its not cost effective to strip the entire car and replace the sills, creating something close to a factory repair. Most likely, what you will be paying for is a patch up, as much rust as possible will be cut out, and new metal will be welded in place.

To contrast, my 17 year old Yaris (with 190,000 miles) has had the start of sill rust (flagged by the MOT last year), I patched it up and applied loads of Waxoyl but it's by no means a perfect repair, once this kind of thing starts, its destined for the scrap heap. Unless of course you're owning a valuable classic or performance car, in which case it probably deserves a complete strip down and sills replaced with new ones... for a Focus, my advice is either sell it now or keep it until it fails MOT and get something newer. You might try cleaning it up with a wire brush and applying some products but it's a losing battle. 13 years of age is quite young to show rust but then it is a Focus and you're in the UK. There are also precautions you can take to prevent this on other cars, make sure drain holes are unblocked and pressure wash the underside of the car and wheel arches often.

  • Thanks. You're telling me what I suspected might be the case (though I hoped it wouldn't be). I think I should probably walk away from this one. – John Rennie May 10 '17 at 10:31
  • Ah, I just realised that this isn't your car yet and you are considering buying it. Yes, this would definitely make me walk away from it because if I owned it I would be thinking about getting rid of it. I bet this is part of the reason for the owner wanting to sell it. – DizzyFool May 15 '17 at 14:53
  • Thanks. The auction finished this morning and yes I did walk away from it :-) – John Rennie May 15 '17 at 15:10
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The inner sills are a vital structural part of the vehicle much more so than the outer sills which are the lower parts of the body below your doors usually from where the front wing/door meet right back to your rear wheel arch.

If it's an advisory then it hasn't rotted or rusted through the sill which is an instant fail but it does depend on whether rusty means surface rust in the paint, underseal or if rust has actually set into the sill.

As there's no mention of the outer sill it suggest it's in the section to rear of the floor pan and just forward of the rear wheel arch because any further forward the inner sills tend to be hidden by the floor pan and outer sill.

Normally for the inner sill at that part of the chassis to be visible to the tester would be because the outer sills are holed.

No part of the inner sill being rusty is good news but at least if it is in the exposed part it can be got to.Again any further forward would require removing the outer or stripping the interior and carpets to get at it from inside the car.

Send the seller a message asking for more details as to whether it's just rusty or actually corroded.

If rusty you can probably strip it back to metal, treat it with cure rust or something similar then prime and paint it then waxoyl it after.

If corroded then it will need to have the corroded parts cut out and new metal welded in. If you can weld it can be done but to pay a garage to do it would be several times what you would spend on the car and that's if they would do the work. Many garages would not consider it worth it.

  • Thanks Bob. I did ask the seller but he said he didn't know. I've decided to to walk away. It seems a shame as the car looks very nice otherwise, but there are lots of second hand Ford Focuses out there so why take the risk? – John Rennie May 14 '17 at 6:52

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