Car sills, or rocker panels, are vital parts of your vehicle’s structure. Positioned just below the doors, they support the car’s frame and help maintain alignment while also offering protection from side impacts and corrosion. If your car has begun to show signs of rust or damage in the sill area, it is important to act quickly before the problem spreads or affects MOT safety standards.
This guide will walk you through each stage of the sill repair process at home, focusing on safety, rust prevention, body alignment, and correct panel fitment.
Step 1: Prepare The Area Around The Damaged Sill
Start by parking the car on a flat, stable surface. Jack up the vehicle and place axle stands underneath the chassis rails to support it securely. Never work under a vehicle supported by a jack alone. For sill repair, you will usually need to remove the front and/or rear wheels depending on which section is being worked on. Make sure you are properly dressed and protected. Sill work involves grinding, welding, and working with rusted metal, which can be hazardous. Wear a long-sleeved shirt, protective gloves, safety goggles or a face shield, and a dust mask or respirator.
Next, mask off nearby panels to prevent damage from sparks, overspray, or adhesive splatter. Use masking film, cardboard, or newspaper to shield surrounding areas, including lights, trim, and painted panels. Finally, clean the sill thoroughly using panel wipe, brake cleaner, or a degreaser. Remove all dirt, wax, underseal, and loose paint so you can clearly assess the damage and ensure good adhesion later in the process.
Before moving to the next step, check the following:
- The car is safely and securely supported
- You are wearing full PPE
- Nearby panels are protected
- The work area is clean and dry
Step 2: Assess & Mark the Damaged Area
Use a wire wheel or flap disc on your grinder or drill to strip away paint and any remaining underseal. You may find that what appeared to be minor surface rust is actually a much larger area of corrosion once the outer layer is removed. Examine the metal carefully. If it crumbles, feels thin under light pressure, or has visible holes, it needs to be cut out. Mark the perimeter of the affected section using masking tape or a marker, leaving a margin of 10 to 15 mm beyond the last visible sign of rust. This ensures you will be welding onto clean, solid metal.
Step 3: Cut Out The Rusted Section
Using an angle grinder with a 1 mm cutting disc, cut along your marked lines in short, careful strokes. Let the tool do the work and avoid pressing too hard. If the damaged section is part of a spot-welded area, use a spot weld drill bit and centre punch to remove the old panel without damaging the surrounding structure. Once the rusted section is removed, smooth the edges with a flap disc. This prepares the surrounding metal for welding and removes any sharp burrs.
Safety tips when using an angle grinder:
- Always use both hands
- Keep body parts well clear of the disc
- Be aware of disc rotation direction
- Never use damaged or worn-out discs
Step 4: Test Fit & Trim The Replacement Sill
Take your replacement sill panel and hold it up to the opening. In most cases, it will need to be trimmed or reshaped to ensure a close fit. Use tin snips or the angle grinder to trim gradually. Do not remove too much at once. Temporarily fix the panel in place using clamps or self-tapping screws. You are aiming for a snug fit that follows the contours of the original sill and matches the existing body lines. If needed, use panel beating tools to reshape any high or low spots.
Key points for a clean fit:
- Compare the new panel against the old one
- Trim gradually, checking alignment frequently
- Match body lines and curves as closely as possible
Step 5: Weld Or Bond The New Panel in Place
There are two common methods for securing the new panel: panel bonding adhesive or MIG welding.
Option 1: Panel Bonding (simpler, ideal for beginners)
Clean both contact surfaces thoroughly using panel wipe. Apply a continuous bead of polyurethane adhesive or panel bond along the edges, press the panel into place, then clamp or screw it temporarily. Leave to cure as per the adhesive manufacturer’s instructions, usually 24 hours.
Option 2: MIG Welding (stronger, preferred for structural repairs)
Start with a few tack welds to hold the panel. Then weld short sections, about 25 mm at a time, moving around the panel to avoid heat distortion. Allow each section to cool before continuing. Once the panel is fully welded, grind the welds flush using a flap disc.
Tips for better welds:
- Use a low voltage setting to avoid blowing through
- Space out welds evenly and rotate around the panel
- Keep a damp cloth nearby to manage heat buildup
Step 6: Protect The Repair Against Rust & Moisture
Once the panel is secure and any welds are ground flat, protect the repair using rust-proofing products.
- Apply a coat of zinc-rich primer to all exposed metal
- Seal all joints and seams using polyurethane seam sealer
- Follow up with a layer of stone chip protection or underbody coating
If the sill encloses an internal cavity, use a probe to inject cavity wax inside to protect from internal corrosion.
Step 7: Fill, Sand, & Prepare For Painting
Mix a small amount of body filler and apply it across the join with a flexible spreader. Use overlapping, smooth strokes and avoid building up excessive layers.
Once cured, sand with 80 grit to shape, followed by 180 and 320 grit to smooth. Feather the edges into the surrounding bodywork. Run your hand across the surface to check for imperfections. A good filler finish should be seamless once painted.
Step 8: Prime, Paint, & Blend The Repair
Wipe the repair area clean with a tack cloth. Apply an etch primer to all bare metal, followed by two or three coats of high-build primer. Allow proper flash times between coats and sand lightly between applications with 600 grit. Spray the topcoat in light, even passes. Avoid heavy coats which may run or cause orange peel. If your paint is metallic or pearl, finish with a clear lacquer. Follow drying times as per the paint manufacturer’s instructions.
Each layer has a purpose:
- Etch primer: promotes adhesion and prevents rust
- High-build primer: fills small imperfections
- Topcoat: provides colour and weather protection
- Clear coat (if used): adds gloss and UV resistance
Step 9: Final Checks & Reassembly
Once the paint is fully dry, apply additional cavity wax if necessary. Refit any trim or components that were removed. Lower the vehicle, stand back, and inspect your work.
A well-executed sill repair should:
- Blend with the surrounding panels
- Show no visible gaps or seams
- Feel solid when tapped
- Keep the car aligned and doors closing properly
Additional Troubleshooting Tips
Rust returns within weeks
Cause: poor preparation or thin primer. Ensure metal is rust-free, primed and sealed properly.
Panel sits unevenly
Check alignment and fitment before final welding or bonding. Remove and refit if needed.
Doors do not close properly
The car may have shifted while jacked up. Always perform an alignment check before welding, and re-check after the repair.
Not Sure You’re Ready?
If you are unsure about welding or vehicle alignment, do not hesitate to consult a professional bodywork technician. DIY repairs are cost-effective, but safety and precision are critical when working on structural components.