Bodywork Basics

Achieve neat and structurally-sound repairs using basic tools

How to patch your classic’s crusty sills properly.

Cardboard, glassfibre, expanding foam, oil containers, chicken wire, newspaper, papier-maché and wood. All of these materials have historically been regarded as fair game in the quest to avoid replacing rusty sills. Once some tiling grout had been used to profile the repair and underseal had been trowelled on to protect the vehicle’s innocence, who was to know? These days, however, the availability of cheap MIG welding sets means there’s no good reason to not do the job properly. A neat, structurally-sound and future-proof welded repair is likely to be quicker, cheaper and easier than any of the above.

Most cars are made from mild-steel, whose goal in life, seemingly, is to revert back to a mineral state. A car’s sills will most likely rust from the inside out and from the ends inwards. Internal corrosion occurs when moisture condenses on the cold steel and trickles down to its lowest points (and hopefully out via silt-free drain holes). Sunroofs often drain into the sills, too. Ends rot due to salty road dirt collecting in the flanges of the wheelarch bottoms.

Sills are a key MOT point. In general, the car will fail if there’s any rot that affects its structural integrity or that’s within a 30cm sphere of a steering, suspension, seat belt or brake mounting. The test stipulates that patches are continuously-welded, though whole panels may be spot- or plug-welded if that’s how the manufacturer fitted them. A patch that incorporates an originally spotwelded seam is a grey area, but it’s likely to be fine if it’s carried out to a high standard.

In this feature, we’re looking at how to assess sills, cut out the damage and repair them properly using basic tools and a MIG welding set. Allow yourself plenty of time. Rust often spreads further than anticipated, so don’t be surprised if you have to tackle inner sill and wheelarch repairs before addressing the outer sills. Don’t start the job the night before the MOT test…

Angle-grinder with cutting, linishing and poly-discs, marking and measuring tools, cardboard and scissors, drill, old wood chisel, welding set and PPE. Try to find an illustration of your car’s sill assembly, so you know where you should and shouldn’t cut with an angle-grinder. The metal thickness will probably be about 1.0mm – so 0.6mm wire, contact tip and guide rollers are ideal. If you find your car has multiple outer sills, you really need to escalate the job. Removing the sills and starting again is the only sensible solution.