MJ Welding and febrication

How to Tell If Your Car Sills Are Rusted (Before It Fails MOT)

Sill rust is one of the most common — and most commonly missed — causes of MOT failure in the UK. Sills sit low down, often hidden behind trims, plastic covers, or a layer of road grime, which means corrosion can spread for months before a car owner even notices anything’s wrong. By the time it’s obvious, you’re often looking at a bigger repair than if it had been caught early.

At MJ Welding & Fabrication in Lye, Stourbridge, sill rust is one of the most frequent reasons cars come to us for a structural welding repair. In this guide, we’ll show you exactly how to check your own car’s sills, what rusted sills look and sound like, why they cause MOT failures, and what to do if you find a problem — before it turns into a much bigger bill.

What Are Car Sills and Why Do They Rust?

The sills are the structural panels running along the bottom edge of your car, underneath the doors, connecting the front and rear of the vehicle’s body. They’re a key part of the car’s structural integrity — they help hold the shape of the body, support the doors, and in many vehicles act as jacking points. Because they sit so close to the road, they’re constantly exposed to moisture, road salt, grit, and stone chips, which is exactly the combination that causes rust to take hold.

Sills often have an outer panel (visible) and an inner panel (structural), separated by a cavity. Rust frequently starts on the inside of this cavity, invisible from the outside, which is why sill corrosion so often goes unnoticed until it’s advanced. You can see how this fits into wider structural issues on our structural welding service page and our guide to professional structural welding.

6 Signs Your Car Sills Are Rusted

1. Bubbling or Flaking Paint Along the Sill

One of the earliest visible signs is paint that’s bubbling, blistering, or flaking along the sill line. This happens when rust forms underneath the paint and pushes it up from below. It might look minor, but it usually means corrosion has already started eating into the metal underneath.

2. Visible Holes or Crumbling Metal

If you can see actual holes, or metal that looks flaky and crumbly rather than solid, the corrosion has progressed well beyond the surface. At this stage, the sill has likely lost a meaningful amount of its structural strength.

3. A Dull “Thud” Instead of a Solid “Knock” When Tapped

A simple (careful) way to check is to gently tap along the sill with your knuckle or a small tool. Solid, healthy metal gives a firm, solid sound. Rusted or thinning metal often sounds duller, hollower, or produces a slight give under light pressure — a sign the metal underneath has weakened.

4. Rust Streaks Running Down From the Sill or Wheel Arch

Orange-brown streaks running down the sill, especially near the wheel arches or jacking points, are a strong sign that corrosion is active somewhere above or within the sill structure, with rust runoff tracking down the panel.

5. Doors That Feel Looser or Don’t Close as Firmly

Because sills support the door frame and body shape, advanced sill corrosion can very occasionally cause doors to feel slightly different when closing — less solid, or with a new gap that wasn’t there before. This is usually a later-stage sign rather than an early one, but it’s worth noting if you spot it.

6. An MOT Advisory or Failure Mentioning Sills, Structure, or Corrosion

If your last MOT certificate included an advisory about sill corrosion, don’t treat it as something to ignore until next year. An advisory means the tester spotted the early stages of a problem that’s very likely to become a failure at your next test — sometimes sooner. Our guide on why cars fail their MOT on corrosion and our page on car failed MOT on corrosion both explain what testers are specifically checking for.

Why Sill Rust Leads to MOT Failure

Sills are classed as a structural, load-bearing part of the vehicle, which means MOT testers assess them very differently from cosmetic bodywork. According to independent guidance summarising the official MOT inspection standards, corrosion becomes a failure point when it weakens a structural or load-bearing area, affects a zone within 30cm of a critical mounting point such as suspension or seatbelt anchors, or reduces the rigidity of the car’s structure. Sills tick several of these boxes at once, which is exactly why they’re one of the most heavily scrutinised areas during testing.

How to Check Your Own Car’s Sills at Home

You don’t need to be a mechanic to do a basic sill check before booking an MOT. A few simple steps:

Get down low and look along the sill line in good light — bubbling paint, discolouration, or visible holes are all worth investigating further.
Run your hand along the underside of the sill (carefully, wearing gloves) to feel for flaking, roughness, or soft spots.
Gently tap along the sill with a knuckle or the handle of a screwdriver, listening for changes in sound that suggest thinning metal.
Check the wheel arches and jacking points, since rust often spreads from these areas into the sill itself.
Look underneath, if you can safely access it, for rust streaks or visible perforation on the inner sill panel.

If you find anything that concerns you, our pre-MOT structural rust checklist covers a broader set of areas worth checking before you book your test, and our guide to 10 hidden signs your car needs welding covers related warning signs beyond just the sills.

What to Do If You Find Rusted Sills

If you spot any of the signs above, the next step is a professional inspection — not a guess based on what’s visible from the outside, since sill corrosion very often extends further than it first appears. At MJ Welding & Fabrication, we assess the extent of the damage properly before recommending a repair. Depending on severity, this might mean:

A partial sill repair, cutting out and replacing the corroded section while leaving sound metal in place — covered in detail on our sills and arch welding service page and our sills and arch welding Stourbridge page.
A full sill replacement, where the entire panel is replaced if corrosion is extensive.
Related structural work, if rust has spread into the chassis rails or floor pan — see our chassis repair service and chassis repair Stourbridge page for more.

Most sill repairs use MIG welding on standard steel, which you can read more about on our MIG welding services page and our MIG welding for cars and automotive page. If you’re unsure of the difference between welding processes, our comparison guide on TIG vs MIG welding for cars explains which one applies to which repair.

How Much Does Sill Rust Repair Cost?

Costs vary depending on how much of the sill has been affected. A small patch repair is generally the most affordable option, while a full sill replacement — particularly if corrosion has spread into adjoining structural areas — will cost more. For a full breakdown of typical UK pricing across different structural repairs, see our 2026 car welding price guide.

Preventing Future Sill Rust

Once a sill has been properly repaired, a few habits can help slow down future corrosion:

Rinse the underside of your car regularly in winter to remove road salt
Keep drainage holes around doors and sills clear so water doesn’t pool
Touch up stone chips and scratches near the sills promptly
Have your underbody and sills checked annually, even outside of MOT season
Consider underseal or cavity wax treatment after a structural repair

More Warning Signs Beyond the Sills

Sill rust rarely appears in complete isolation — it’s often part of a wider pattern of corrosion affecting other structural areas. Our guides on the 5 most common car welding repairs and 7 signs your car needs structural welding repair right now both cover related issues worth checking at the same time as your sills, including chassis corrosion, arch rust, and floor pan damage.

Areas We Cover for Sill Rust Repair

We carry out sill welding repairs for customers across Stourbridge and the wider West Midlands and Worcestershire. Our coverage includes:

Around Stourbridge & Dudley:

Halesowen, Dudley, Kingswinford, Wollaston, Norton, Pedmore, Gig Mill, Wollescote, Withymoor, Pensnett, Rowley Regis, Oldbury, Smethwick, Caledonia and Churchill.

Birmingham & surrounding suburbs:

Birmingham, Harborne, Edgbaston, Bearwood, Quinton, Northfield, Bournville, Selly Oak, Kings Heath, Longbridge and Solihull.

Black Country & Staffordshire:

Wolverhampton, Walsall, West Bromwich, Wednesbury, Bilston, Tipton and Penkridge.

Worcestershire & Bromsgrove area:

Bromsgrove, Redditch, Sutton Coldfield, Alvechurch, Shirley, Romsley, Clent, Blakedown, Worcester, Martley and Ombersley.

Shropshire & Wyre Forest:

Bridgnorth, Claverley, Highley, Wyre Forest, Cookley, Shatterford and Tanwood.

This isn’t a complete list — for the full, up-to-date coverage area, visit our service areas page, our car welding near Stourbridge page, or our West Midlands locations hub.

More Guides From MJ Welding & Fabrication

About MJ Welding & Fabrication

MJ Welding & Fabrication is a specialist automotive welding and fabrication workshop based at Bromley St, Lye, Stourbridge, DY9 8HU, with over 15 years of experience in structural, MOT, and corrosion repair. We focus exclusively on welding and fabrication rather than general mechanical work, which means sill, chassis, and structural repairs get our full attention every time. We work on all makes and models, from everyday hatchbacks to premium 4x4s, vans, and camper van conversions, using professional MIG, TIG, and ARC welding equipment. Read more on our About Us page, browse our full range of services, or see our complete list of blog guides.

Frequently Asked Questions

Can rusted sills be repaired, or do they always need replacing?

It depends on the extent of the corrosion. Localised rust can often be cut out and patched, while more extensive corrosion usually requires a full sill replacement to restore proper structural strength.

Will sill rust automatically fail my MOT?

Not always — it depends on severity and location. Surface corrosion may only generate an advisory, but corrosion that weakens the structure or falls within 30cm of a critical mounting point will result in a failure.

How do I know if sill rust has spread further than it looks?

You often can’t tell from the outside alone. A professional inspection, sometimes involving removing sill trims or covers, is the only reliable way to assess the full extent of the damage.

Can I drive with rusted sills?

If the corrosion hasn’t yet caused an MOT failure, the car may technically still be driveable, but it’s worth having it assessed promptly, since sill rust tends to worsen quickly once it takes hold.

How long does a sill repair take?

A partial patch repair can often be completed within a day. A full sill replacement typically takes one to two days, depending on the extent of the damage and any related structural work required.

Does sill rust affect my car’s resale value?

Yes, particularly if it’s undisclosed or left untreated. A properly documented, professionally completed repair generally has far less impact on value than an ignored corrosion issue.

Book a Sill Inspection in Stourbridge Today

If you’ve spotted any signs of sill rust, or your last MOT flagged a corrosion advisory, don’t wait for it to progress into a failure. Contact MJ Welding & Fabrication today for an honest inspection and a clear, upfront quote.

📞 Call: +44 7532 794934 📍 Bromley St, Lye, Stourbridge DY9 8HU