Rear Sill Corrosion on Mk3 Fords

The Insidious Spread of Rear Sill Corrosion on the Mk3 Ford

The Mk3 Ford, a beloved model for many enthusiasts, often carries with it the hallmarks of its era, and unfortunately, this can include a susceptibility to rust. One of the most common and visually detrimental areas affected is the rear sill. These structural components, vital for the integrity of the car's bodywork, are particularly vulnerable due to their position and the way they channel water and road debris. Identifying and addressing this corrosion early is paramount to preserving your Mk3 and preventing costly repairs down the line. This article will delve into the tell-tale signs of rear sill corrosion on your Mk3, explore the common causes, and outline effective strategies for inspection, prevention, and repair.

Understanding the Mk3 Rear Sill and Why it's Prone to Rust

The sill, or rocker panel, runs along the lower edge of the car's body, between the front and rear wheel arches. On the Mk3 Ford, like many vehicles of its vintage, the design of the rear sill often incorporates multiple layers of steel. These layers, while providing structural strength, also create numerous cavities and pinch welds where moisture can become trapped. Furthermore, the sill is exposed to constant spray from the road, including water, salt (especially in regions where de-icing salt is used), mud, and grit. Over time, any small nicks or scratches in the paintwork or underseal can allow moisture to penetrate these vulnerable areas, initiating the corrosive process. The trapped moisture, combined with the electrolytic action of salts, accelerates the rusting of the steel.

Identifying the Signs of Rear Sill Corrosion

The early stages of rear sill corrosion can be subtle, but with a keen eye, you can often spot the warning signs before they become a major problem. Here's what to look for:

• Bubbling Paintwork: This is often the first visible indicator. Small blisters or bubbles appearing on the surface of the sill, particularly around the lower edges, wheel arches, and jacking points, suggest that rust is forming underneath the paint.
• Surface Rust: As corrosion progresses, the bubbling may rupture, revealing patches of orange or reddish-brown rust on the metal surface.
• Perforation: In more advanced stages, the rust will eat through the metal entirely, creating holes. You might notice these by poking gently with a screwdriver (be careful!) or by observing daylight through the sill. This is a critical stage requiring immediate attention.
• Swollen Underseal: If the original underseal is intact but bulging or cracking, it can indicate trapped moisture and rust developing beneath it.
• Damp Carpets: Water ingress is a common consequence of severe sill corrosion. If you notice dampness or mould growth on the carpets inside the car, especially near the rear seat area, it's a strong indicator that the sills may have compromised integrity.
• Door Fitment Issues: In extreme cases, severe rust and structural weakening of the sills can lead to the car's body flexing, causing doors to become difficult to open or close, or misaligned.

Common Causes of Mk3 Rear Sill Rust

Several factors contribute to the likelihood of your Mk3 developing rear sill corrosion:

• Age and Material: As mentioned, the Mk3 is an older vehicle. The steel used in its construction, while robust for its time, may not have the same advanced rustproofing treatments as modern cars.
• Design Vulnerabilities: The multi-layered construction and potential for trapped moisture within the sill cavities are inherent design weaknesses that are difficult to overcome without significant modification.
• Stone Chips and Scratches: Minor impacts from stones or debris on the road can chip away at the protective paint and underseal, exposing the bare metal to the elements.
• Salt and Grime Accumulation: The constant exposure to road salt, especially during winter months, significantly accelerates the rusting process. Even if the salt is washed off, it can leave behind residual moisture and corrosive elements.
• Blocked Drainage Channels: Many vehicles have small drain holes in the sills to allow water to escape. If these become blocked with leaves, mud, or debris, water can stagnate within the sill, leading to rapid corrosion.
• Poor Repair Work: If the car has had previous bodywork, especially repairs to the sills, inadequate preparation, use of the wrong materials, or poor welding can create new areas for rust to take hold.

Inspection: A Proactive Approach

Regular inspection is your best defence against severe rear sill corrosion. Make it a habit to check your Mk3's sills at least a couple of times a year, perhaps during your regular washes or service checks.

What to do during an inspection:

1. Visual Examination: Get down low and look closely at the entire length of the rear sills. Pay particular attention to the areas mentioned above: lower edges, wheel arch lips, jacking points, and where the sill meets the front and rear wings.
2. Tap Test: Gently tap the sill with a plastic-headed hammer or the handle of a screwdriver. Listen for any dull, tinny sounds, which can indicate that the metal is rusted through and losing its rigidity. A solid thud suggests healthy metal.
3. Probe Carefully: If you suspect corrosion, use a blunt, pointed object (like a stiff plastic trim tool or a dull screwdriver) to gently probe any suspect areas. Be extremely cautious not to force it or cause further damage if the metal is weak. The aim is to ascertain the extent of any perforation.
4. Check Drainage Holes: Locate the drain holes (usually at the lowest points of the sills, often near the jacking points) and ensure they are clear. You can use a piece of wire or compressed air to dislodge any blockages.
5. Inspect Underseal: If the underseal looks suspicious, try to gently peel back any loose sections to reveal the metal underneath.

Prevention is Key

While you can't completely eliminate the risk of rust on an older car, you can significantly slow its progress and prevent new corrosion from forming.

• Regular Washing: Keep your Mk3 clean, especially during winter. Thoroughly wash the underside, including the sills, to remove road salt and grime.
• Underseal and Waxoyl: Consider applying a good quality underseal or cavity wax (like Waxoyl) to the sills and other vulnerable areas. This creates a protective barrier against moisture and salt. For the cavities, injecting a cavity wax is highly recommended.
• Touch-Up Paint: Immediately repair any stone chips or scratches on the sills to prevent moisture from reaching the bare metal.
• Keep Drainage Holes Clear: As mentioned, regularly check and clear the sill drain holes.
• Garage Storage: Storing your Mk3 in a dry garage whenever possible will protect it from the elements.

Repairing Rear Sill Corrosion

The repair process for rear sill corrosion depends heavily on the extent of the damage.

Minor Surface Rust:

1. Clean the affected area thoroughly.
2. Remove all loose rust using wire brushes, sandpaper, or a wire wheel on a drill.
3. Treat the cleaned metal with a rust converter or rust inhibitor.
4. Prime the area with a good quality metal primer.
5. Fill any minor pits with body filler.
6. Sand smooth and paint to match the car.
7. Apply underseal once the paint is cured.

Perforated Metal (Holes):

This is where repairs become more involved and often require professional expertise. The process typically involves:

1. Cutting Out Rusted Metal: The rusted sections of the sill must be cut out completely. This often means cutting back to clean, solid metal.
2. Fabricating or Sourcing Replacement Panels: New metal patches or sections of sill will need to be fabricated or sourced. For Mk3s, specialist suppliers may offer repair panels.
3. Welding: The new metal is then carefully welded into place. This requires good welding skills to ensure a strong, rust-resistant join. MIG welding is commonly used.
4. Grinding and Smoothing: The welded seams are ground smooth to blend with the surrounding bodywork.
5. Rust Treatment: All cut edges, internal surfaces of the sill, and the new metal are treated with rust inhibitors and primed.
6. Sealing and Undersealing: The internal cavities of the sill should be filled with cavity wax. The exterior is then treated with primer, paint, and finally, underseal.

Important Considerations for Repair:

• Structural Integrity: The sills are a crucial part of the car's structure. Any repairs must restore this integrity. Cutting out and welding in new metal is the only truly effective long-term solution for perforation.
• Professional Help: If you are not experienced in metal fabrication and welding, it is highly recommended to seek professional help from a reputable body shop. Poorly executed repairs can be worse than no repairs at all.
• Cavity Protection: Ensure that the internal cavities of the sills are thoroughly treated with cavity wax after any repair, and ideally, as a preventative measure on healthy sills.

Frequently Asked Questions

Q1: How serious is rust on my Mk3 rear sill?

A1: Even small amounts of rust can spread quickly. Perforation means the structural integrity of the sill is compromised, which is a serious issue that needs immediate attention.

Q2: Can I just cover the rust with filler?

A2: No, this is a temporary fix at best and will allow the rust to continue spreading underneath. The rusted metal must be removed.

Q3: What is the best way to protect my Mk3 sills from rust?

A3: Regular cleaning, keeping drainage holes clear, and applying a good quality underseal and cavity wax are the most effective preventative measures.

Q4: How much does it cost to repair rusted sills?

A4: This varies greatly depending on the extent of the rust. Minor surface rust repairs can be relatively inexpensive, while cutting out and welding in new metal can cost several hundred pounds per side, or more if significant fabrication is needed.

Conclusion

The rear sills on a Mk3 Ford are a common place for rust to develop, but with diligent inspection, preventative maintenance, and timely repairs, you can keep this classic car in excellent condition. Don't let corrosion undermine the structural integrity and aesthetic appeal of your Mk3. By understanding the signs and taking action, you can ensure your beloved Ford continues to turn heads for years to come.

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