Car sill welding costs vary widely because the rust you can see is often only the outer edge of the damage. In the UK, a small localised repair may cost a few hundred pounds, while replacing corroded inner and outer sill sections can cost well over £1,000 per side. The final price depends on access, vehicle construction, paintwork, corrosion protection, and what the workshop finds after cutting back to solid metal.
Quick Answer
In the UK, budget about £200–£700 for a localised welded sill repair and £800–£1,500+ per side when inner and outer sections need replacement. Surface treatment without welding may cost £80–£300. Two-sided or linked arch, floor, and jacking-point repairs can exceed £1,500, especially when hidden corrosion is found.
Key Takeaways
- A sill repair can range from a small welded patch to replacement of the inner sill, outer sill, reinforcement, floor edge, or jacking point.
- Sill rust is not automatically an MOT failure, but corrosion that weakens a prescribed or load-bearing area can be classed as major or dangerous.
- Ask whether the price is per side and whether it includes VAT, panels, fabrication, paint, seam sealer, underbody protection, cavity wax, and an MOT retest.
- A written estimate should set an approval limit for extra work because the full damage may only become visible after strip-down.
- Structural sill welding is advanced safety-critical work. A competent vehicle body repairer should follow the manufacturer’s repair method and current MOT repair guidance.
At a Glance
| Time Required | About 1–3 working days for a local repair; around 3–7 days or longer for inner-and-outer sill work, paint, and corrosion protection |
| Difficulty | Advanced and safety-critical when the sill is structural or close to seat-belt, suspension, steering, or braking mountings |
| Tools Needed | Professional lifting equipment, cutting and panel-forming tools, the correct joining equipment, measuring tools, fume extraction, fire protection, and finishing materials |
| Cost | Roughly £80–£300 for non-welded surface treatment, £200–£700 for localised welding, and £800–£1,500+ per side for major sill reconstruction |
What Affects Sill Welding Cost?
The visible rust patch is only one part of the quote. A workshop must identify how far the corrosion extends, what the sill does in that vehicle’s structure, and whether nearby panels have also weakened.
- Repair scope: Treating surface rust costs less than cutting out perforated steel and fabricating a replacement section.
- Inner versus outer sill: An outer skin repair is usually simpler than rebuilding the inner sill, reinforcement, floor edge, or jacking point.
- Access and strip-down: Seats, carpets, trim, wiring, fuel lines, plastic foam, and underseal may need removal or protection before hot work.
- Vehicle design: Some models have readily available repair panels. Others need hand-formed sections or manufacturer-specific joining methods.
- Finish level: An MOT-focused structural repair may cost less than a restoration-quality repair with invisible joins and colour-matched paint.
- Corrosion protection: Primer, seam sealer, paint, underbody coating, and cavity wax add cost but help protect the new metal.
- Location and VAT: Labour rates vary across the UK. Confirm whether the written price is per side and includes VAT.
The hourly rate alone does not predict the bill. Access, strip-down, fabrication, paint, and hidden inner-sill corrosion often matter more than the welding time.
Is Sill Rust Cosmetic or Structural?
A sill can include a visible outer skin, an inner sill, reinforcements, jacking-point structure, and adjacent floor sections. Whether corrosion is cosmetic or structural depends on which part is affected and how the vehicle is built. A plastic sill cover may be cosmetic, while the steel behind it can still be load-bearing.
Surface vs Structural Rust
Surface rust affects the coating and the top layer of metal. The steel remains firm, with no perforation or major loss of thickness. A body shop may be able to remove the corrosion, treat the metal, prime it, and repaint it without welding.
Structural corrosion has caused holes, soft metal, swelling between joined panels, failed seams, or a serious reduction in section thickness. The damaged steel must usually be removed back to sound metal before a suitable repair section is installed.
Hidden Sill Damage
Sills often corrode from the inside because moisture and debris collect in seams and cavities. Bubbling paint may therefore hide damage to the inner sill or reinforcement. A proper assessment may include a lift inspection, removal of trim or plastic covers, examination of drain holes and jacking points, and a small borescope where access allows.
Do not stab a rusty sill with a screwdriver or rely on a household magnet as a diagnosis. Paint thickness, filler, plastic covers, aluminium panels, and panel shape can all affect a magnet test. The current DVSA method uses visual inspection, finger-and-thumb pressure, and careful use of an approved corrosion assessment tool by the tester.
Warning: Never lift the vehicle from a jacking point that is swollen, perforated, cracking, or visibly distorted. Use the manufacturer’s alternative lifting point or arrange recovery and professional inspection.
Will Sill Rust Fail an MOT?
Not every rusty sill fails an MOT. Under the DVSA structural integrity and corrosion guidance, the result depends on the location, extent, and structural function of the affected metal.
A load-bearing or supporting area within 30cm of certain brake, steering, suspension, axle, or seat-belt mounting points is a prescribed area. A hole, soft metal, or an inadequate repair in such an area can lead to rejection. Corrosion outside a prescribed area can also fail if it significantly reduces the strength or continuity of the overall vehicle structure.
A repair does not receive an automatic pass merely because fresh metal covers the hole. Structural work must be properly carried out, extend to sound metal, use suitable materials, and appear as strong as the original structure. Patch repairs normally require continuous seam welding unless an approved original or manufacturer-recommended method applies.
Signs Your Sill Needs Welding
These signs justify a professional inspection. They do not all prove that welding is required, but the risk rises when several appear together.
| Sign | What It May Mean | Next Step |
|---|---|---|
| Brown staining or light scaling | Early coating failure or surface corrosion | Book rust treatment before it spreads |
| Bubbling paint or swollen seams | Corrosion may be developing behind the outer skin | Inspect the inner sill and drainage path |
| Flaking metal, pinholes, or perforation | Metal thickness and strength have been lost | Arrange structural assessment promptly |
| Crushed or collapsing jacking point | The reinforcement or adjacent sill may be weak | Do not jack there; have the area inspected |
| Cracked seam sealer or fresh underseal over a lumpy repair | Moisture entry or a previous repair that needs checking | Ask for an underside and cavity inspection |
| Door opening distortion or unusual creaking | Possible structural movement, although hinges or accident damage can cause similar symptoms | Stop guessing and obtain a body-structure assessment |
Note: A damp footwell can come from door membranes, windscreen seals, heater drains, roof drains, or floor corrosion. It is a reason to investigate water entry, not proof that the sill needs welding.
Products Worth Considering
UPGRADED 150W RAPID HEATING - This 150W plastic weld adopts advanced ceramic heating chip, It can be heated to 1112 ℉ in 15-20 seconds. By quickly heating, melting, and repairing, the plastic welding gun reassembles broken plastic to extend the lifespan of items, saving a significant amount of time and money. Compared with the traditional tungsten wire chips, it not only heats up faster, but also lasts 10 times longer than the traditional tungsten chip. And this plastic welding with 6 ventilation holes, it helps to dissipate heat quickly. The design of plastic handle insulation is not easy to deform and don't worry about burning your hands.
Package includes: you will receive 78 pieces of plastic welding rods in various colors, such as black, white, gray, transparent, etc., each color counting 12 pieces except for 6 pieces of transparent poly-carbonate rods, which are sufficient quantity and diverse varieties to meet your multiple demands
Premium Material: This automotive bumper repair machine employs a combination of premium pure copper and durable plastic, ensuring long-term reliable operation. The pure copper transformer heats rapidly, distributing heat evenly across the welding area to minimise cold spots that could compromise weld strength or damage plastic. The plastic casing offers triple protection: high-temperature resistance, cushioning against accidental drops, and shielding internal components from dust ingress.
How Sill Welding Is Done
A lasting repair removes weakened metal, restores the original load path, seals the joins, and protects both sides of the new steel. The exact process depends on the vehicle manufacturer’s repair information and the construction of the sill.
Products Worth Considering
Multipurpose Fit: Repairs at least 98% of the plastics on the market. Suitable for repairing: bumper, dashboard, lamp holder, plastic ring, radiator, spoiler, handles, headlights, plastic wings, car and motorcycle plastic device, etc
220W Rapid Heating:The plastic welding machine has a power of 220 watts and a 110V-US plug, which has higher output current and faster heating speed. Using heat to embed nails or plastic rods into plastic will not cause damage to the car or plastic surface, and is very sturdy. Not only are there two repair methods to choose from: welding rods and hot nails, but there is also an overheat protection device to ensure your safety
UL CERTIFIED: Our Plastic Welder, the factory, and the process have been tested, approved, and Certified by Underwriters Laboratories (UL). Safety is our number one priority
1. Inspection and Strip-Down
The repairer checks the outer sill, inner sill, reinforcement, jacking points, floor edge, wheel arch, and nearby mounting areas. Trim, carpets, wiring, sealant, and coatings may need to be moved so the full repair area and fire risks are visible.
Before hot work, a competent workshop should assess nearby fuel lines, the fuel tank, upholstery, plastic foam, underseal, and other flammable materials. The Health and Safety Executive’s vehicle welding guidance highlights fire, explosion, electric-shock, ultraviolet-radiation, and harmful-fume risks.
2. Cutting and Fabrication
The technician marks the full corrosion boundary and cuts back to firm, clean metal. Covering rust with a plate, body filler, or underseal does not restore the original structure. A repair panel is then selected or fabricated to match the shape, thickness, overlaps, drainage, and load path of the original section.
- Support and measure the body before removing a structural section.
- Remove all weakened metal rather than welding onto thin edges.
- Trial-fit the panel and confirm door gaps, jacking-point position, and nearby mountings.
- Prepare mating surfaces and preserve drain paths and cavity access.
3. Joining the New Metal
MIG welding is common on older mild-steel body repairs, but it is not the only acceptable method. The correct process may include spot welding, plug welding, continuous seam welding, MIG brazing, riveting and bonding, or a manufacturer-specified combination. The material and joining method must suit the original construction.
DVSA guidance says a structural repair must extend to sound metal and appear as strong as the original structure. A patch repair normally needs a continuous seam weld. Spot, stitch, or plug methods are acceptable in the circumstances described by the inspection manual or where the vehicle manufacturer specifies them.
4. Protection and Finishing
After the structural work is checked, the repair is dressed only as much as necessary, sealed, primed, painted, and protected underneath. Cavity wax is applied inside accessible closed sections after the repair has cooled and the coatings are ready for it. Drain holes must remain open.
The final check should cover weld continuity, panel fit, door operation, jacking-point position, coating coverage, nearby pipes and wiring, and any items removed during strip-down. An MOT retest checks testable standards; it is not a lifetime warranty against rust returning.
Car Sill Welding Cost by Damage Level
The following bands are practical budgeting estimates, not a regulated national tariff. Vehicle model, region, access, finish, parts supply, and hidden corrosion can move a quote outside these ranges.
| Damage Level | Likely Work | Indicative UK Cost |
|---|---|---|
| Surface rust only | Clean, treat, prime, and repaint; no welding | About £80–£300 |
| Small perforation or patch | Cut out a local area and weld in a small repair section | About £125–£400 |
| Localised sill repair | Larger outer-sill section, fabrication, sealing, and local refinishing | About £200–£700 per side |
| Inner and outer sill damage | Rebuild reinforcement, inner sill, outer sill, or jacking-point structure | About £800–£1,500+ per side |
| Multi-panel structural corrosion | Both sides or linked repairs to arches, floor edges, pillars, and jacking points | About £1,500–£3,000+ total |
Ask the garage to label the price as a quote or an estimate. A quote is generally a fixed price for the agreed scope. An estimate is a best assessment that may change if strip-down reveals more corrosion.
What Should a Sill Welding Quote Include?
A low headline price can become expensive if it excludes paint, VAT, or corrosion protection. Compare written scopes rather than comparing one number.
- Whether the price is for one side or both sides
- Whether it covers the outer sill only or the inner sill and reinforcement
- Repair-panel cost, custom fabrication, weld consumables, and clips or trim
- Removal and refitting of seats, carpet, plastic covers, wiring, and other components
- Paint preparation, colour matching, seam sealer, and underbody coating
- Internal cavity wax and clearing of drain holes
- VAT and any diagnostic or strip-down charge
- The expected completion time and whether an MOT retest is included
- A written limit above which the garage must contact you for approval
- Photos of hidden corrosion before extra work and photos of the completed bare-metal repair before it is covered
Pro Tip: Ask for two figures where possible: the visible-damage estimate and a clearly stated worst-case ceiling if the inner sill or adjacent floor is also corroded.
Extra Repairs That Raise the Bill
Sill corrosion often meets other panels at seams, so the repair can expand after the outer skin is removed. Common additions include:
- Inner-sill and reinforcement repairs
- Jacking-point reconstruction
- Floor-edge or cross-member repairs
- Rear dogleg and wheel-arch repairs
- Lower A-pillar or B-pillar work
- Brake-pipe, fuel-line, wiring, clip, or heat-shield replacement where nearby parts are damaged
- Door alignment or previous accident-repair correction
- Removal of old filler, overlapping patches, or trapped rust from an earlier repair
Paintwork can also be a major part of the bill. Blending into adjacent panels for a near-invisible finish costs more than protecting a repair hidden below a plastic trim cover.
Products Worth Considering
Package Including 20 x .035" (0.9mm) contact tips, 2 x flux core gasless nozzles, 2 x gas diffusers
Spiral design: Features a durable spiral design specifically engineered for effective residue removal, ensuring thorough cleaning and superior welding performance.
COMPATIBILITY: Designed for 14AK welding torches, this kit is a direct fit for YESWELDER FLUX-135PRO/FLUX-135MINI models. It’s also compatible with any other welder that uses a 14AK gun. Ideal for gasless welding and general metal fabrication in workshops, construction sites, and automotive repair applications.
Why Sill Welding Quotes Change
Corrosion can spread between layers where it is not visible during the first inspection. Once the workshop removes trim, underseal, filler, or the outer sill, the repairer may find that the planned patch has no sound edge to weld to.
A changing estimate can therefore be reasonable, but extra work should still be explained and authorised. Citizens Advice explains the difference between a quote and an estimate, while The Motor Ombudsman advises keeping written estimates, invoices, authorisation records, and photographs.
Note: Before work starts, agree how the garage will contact you, the maximum extra amount it can approve without a call, and whether you will pay a strip-down or reassembly charge if you decline the expanded repair.
How Long Does Sill Welding Take?
A small accessible patch may be completed within a day, but drying, painting, parts delays, and hidden corrosion can extend the booking. A broad planning guide is:
- Surface treatment without welding: about 1–2 working days
- Local welded section: about 1–3 working days
- Inner-and-outer sill repair with paint: about 3–7 working days
- Both sides or linked restoration work: one to several weeks, depending on fabrication and parts
Ask for a completion range rather than a guaranteed collection time before strip-down. Also ask whether paint curing, cavity treatment, MOT testing, and reassembly are included in that range.
DIY vs Professional Sill Welding
Removing loose dirt, clearing drain holes, washing road salt away, and touching up a small paint chip are reasonable owner-maintenance tasks. Structural sill welding is different. It can affect body stiffness, seat-belt and suspension mounting areas, crash load paths, jacking points, fuel-system safety, and MOT compliance.
A professional body repairer has the lifting equipment, repair information, measuring tools, extraction, fire controls, and experience to identify where sound metal begins. The technician can also choose a joining method that suits the vehicle instead of assuming every sill should be repaired with the same MIG patch.
HSE guidance warns that vehicle hot work can ignite trim, carpet, underseal, plastic foam, fuel lines, tanks, and flammable residues. Welding and sanding can also release harmful fumes and fine dust. Local exhaust ventilation should be used wherever possible and always in confined locations, with suitable eye, skin, electrical, and fire protection.
Warning: Do not attempt structural sill welding as a first welding project. A neat-looking bead does not prove correct penetration, panel thickness, body alignment, corrosion removal, or restored structural strength.
Is Sill Welding Worth the Cost?
Compare the complete repair cost with the car’s value after repair, not only its current damaged value. A repair may make sense when the rest of the vehicle is sound, replacement would cost much more, parts are available, and the workshop can provide a durable documented repair.
Think carefully when corrosion affects both sides, the floor, arches, pillars, subframe mountings, or several previous patches. Also include upcoming tyres, brakes, suspension, timing-belt, transmission, or emissions work in the decision.
- Repair total: welding, fabrication, paint, VAT, MOT, transport, and likely extras
- Post-repair value: realistic private-sale or trade value with invoices and photographs
- Replacement cost: price of a sound alternative car plus inspection, tax, insurance changes, and immediate maintenance
- Expected life: how long the repaired car is likely to meet your needs without another major corrosion job
Spending £4,000 can be sensible on a valuable classic, rare vehicle, or otherwise excellent car. It is harder to justify on a low-value car with widespread corrosion and other major faults. An independent inspection can help separate a worthwhile repair from an open-ended restoration.
How to Stop Sill Rust Returning
A strong weld can still rust again if water enters the cavity or the repair is left unsealed. After the work is complete:
- Confirm that primer, seam sealer, paint, underbody protection, and internal cavity wax were applied where appropriate.
- Keep factory drain holes open; do not seal a cavity so tightly that trapped water cannot escape.
- Wash road salt and packed mud from the sill and wheel-arch area, especially after winter driving.
- Repair stone chips and damaged seam sealer before bare steel remains exposed.
- Inspect jacking points and sill seams during servicing and before each MOT.
- Check model-specific roof, scuttle, or hood drains where blocked drainage can feed water into sill cavities.
- Avoid thick fresh underseal over active rust. The corrosion must be removed or properly treated first.
Ask the repairer whether the cavity treatment needs inspection or renewal and whether the paint or corrosion-protection work carries a written warranty.
Frequently Asked Questions
How do you calculate a sill welding repair price?
A garage normally combines inspection and strip-down time, labour, repair panels or fabricated steel, consumables, paint materials, corrosion protection, VAT, and any MOT retest. Because hidden rust can change the hours and materials, ask whether the figure is a fixed quote or an estimate with an approval limit.
What is usually the most expensive part of sill repair?
Labour and fabrication usually dominate because the technician must strip the area, cut back to sound metal, form and fit sections, protect nearby components, weld or join the panels correctly, finish the surface, and reassemble the car. Paint blending and rebuilding inner structures can add heavily to the total.
How much should car sill welding cost?
A localised UK sill repair often falls around £200–£700 per side. Inner-and-outer sill reconstruction may be around £800–£1,500+ per side, while both sides or linked floor and arch work can exceed £1,500. Confirm exactly what the price includes.
Should I spend £4,000 fixing sill corrosion?
It can make sense if the car is valuable, rare, otherwise reliable, or much cheaper to repair than replace. It is less attractive when corrosion is widespread or major mechanical costs are also due. Compare total repair cost, post-repair value, replacement cost, and expected service life.
Can you drive with rusty sills?
Light surface rust is not the same as structural failure, but holes, soft metal, a collapsing jacking point, or distortion need prompt assessment. Do not drive a vehicle that has been classed as dangerous, and do not use a weakened sill as a lifting point.
Can a rusty sill pass an MOT?
Yes, if the corrosion is not severe enough to meet the failure criteria. It can fail when a prescribed or load-bearing area has a hole, feels soft, is inadequately repaired, or has lost significant structural strength. The exact result depends on location and vehicle construction.
Is a patch repair acceptable on a car sill?
A properly designed patch can be acceptable when it reaches sound metal and restores strength. DVSA guidance normally requires patch repairs to be continuously seam welded unless an approved original or manufacturer-recommended method applies. Filler, fibre reinforcement, or underseal alone is not a structural repair.
How long should a sill welding repair last?
There is no universal lifespan. Durability depends on complete rust removal, correct materials and joining, sealed seams, paint coverage, internal cavity protection, open drains, climate, and aftercare. Ask for photographs and a written workmanship or corrosion-protection warranty.
Conclusion
Car sill welding is not simply a matter of covering a rusty hole. The repair must reach sound metal, restore the correct structure, protect nearby components, and seal the new work against moisture. For budgeting, expect a few hundred pounds for localised welding and £800–£1,500+ per side when inner and outer sill sections need rebuilding. Get a written scope, set an authorisation limit for hidden damage, and choose a repairer who can explain the method, finish, and corrosion protection.
Sources
- DVSA MOT inspection manual: structural integrity and corrosion — prescribed areas, corrosion assessment, failure criteria, and acceptable repair methods
- DVSA MOT inspection manual: body, structure and attachments — major and dangerous structural defects
- Health and Safety Executive: welding and flame cutting in vehicle repair — hot-work, fire, electrical, radiation, and fume controls
- Health and Safety Executive: motor vehicle repair health risks — welding fume, sanding dust, coatings, and local exhaust ventilation
- Citizens Advice: problems with a car repair — quotes, estimates, reasonable prices, and unauthorised work
- The Motor Ombudsman: disputing a car repair bill — evidence, authorisation, invoices, and repair disputes
