Repairing rusty sills on a Mazda MX-5 NA and NB.
Rust in the sills and chassis rails (box sections) is a very common problem with the Mazda MX-5 NA and NB. Fortunately, repair panels are available, and very rusty MX-5s can still be saved!
Why do Mazda MX-5 sills and quarter panels rust?
Rust occurs when (untreated) steel is exposed to moisture for a long time. The sills and quarter panels are a 'prime' example of this; the places where rust always occurs on an MX-5 are the sills on the underside and the wheel arch edges.
These are places where two layers of steel are spot-welded together but still allow space for moisture. Once this moisture gets between these sheet metal parts, it can be difficult for it to escape, giving it every opportunity to corrode the sheet metal until it is completely rusted away.
The photo above clearly shows where the outer skin of the sill and the inner part are in direct contact. The formation of rust is therefore partly due to the construction/design of the sheet metal parts used by Mazda.
Recognizing early rust in a Mazda MX-5
Because rust always starts from the inside out in a Mazda MX-5 due to the construction used, it is difficult to see if rust is starting to form in the known spots.
With the following tip, you can check for yourself if your Mazda MX-5 is starting to get rust on the sills and quarter panels:
Make sure your Mazda MX-5 is clean and then place it in an environment with plenty of light. Thoroughly inspect the wheel arch edges and sills for unevenness in the paint such as: small bubbles, bumps, and strange spots where the paint is starting to peel.
If you find any of these things, it is very likely that rust is present behind the body panel. The rusted sill/quarter panel shown above only had a few bubbles in the paint…
Patching up or expertly repairing the rust?
This is, of course, a rhetorical question, but I will explain it. We regularly receive Mazda MX-5s in the workshop that have been patched up before to pass inspection or as a quick repair.
For the sill shown below, the rust was left in place, a repair panel was welded over the rust, and then everything was neatly finished. From the outside, it looked very clean, but as shown below, the rust from the inside simply continues.
Professional repair is the only correct way
To conquer the rust for many years, at LucaCarMods, we always remove all rust that appears behind the bodywork.
After drilling out the spot welds and cutting through the sheet metal, it is only then truly possible to assess the severity of the situation and which panels need to be replaced. This makes it impossible to provide an exact price beforehand if we want to charge our customers a fair price.
How we work
- Drill out spot welds and grind away the sheet metal up to where the rust is visible from the outside. It may be that the rust is in an early stage and the entire quarter panel does not need to be addressed.
- If the underlying rust turns out to be higher, we grind out more sheet metal to ensure that all rust can actually be dealt with.
- If the underlying sheet metal is superficially rusted, we completely de-rust it and then treat it with a Zinc coating.
However, if the sheet metal has become thin due to rust, the inner sill will also have to be cut out and replaced with a new part that has also been treated with a zinc coating.
This is an exception, however; normally, this is also completely rusted….
- Now that the interior is coated, we cut the new sheet metal for the MX-5 sill to size. For a good end result, it is important that the fit of this panel is very good and that there is a gap of approximately 1-2mm everywhere for welding.
- When welding sheet metal, too much heat should not be introduced into the sheet metal. This can cause the sheet metal to warp, creating folds or bumps.
We do this by moving a few centimeters after 1-2 short tack/spot welds and making another 1-2 short welds here. This ensures better heat distribution.
- After welding, we sand the weld completely flat so that it is barely visible anymore - see photo below -.
For finishing, there are 2 options at LucaCarMods:
1. We make the weld completely smooth, with a very thin layer of filler if necessary, and finish the entire sill with a rough black stone chip coating. As is standard for many 1st generation MX-5s.
2. A zinc coating so that the sill remains rust-free and the customer can take the MX-5 to a painter to have the sill finished smoothly and painted in color.
- Finally, we will always drill a small 10mm hole on the inside of the wheel arch so that cavity wax coating can be applied with a long hose. We use Dinitrol 1000 Cavity Wax for this. This ensures that in addition to the zinc coating, an extra protective layer is applied in the sill.
We seal this hole with a plastic cap after coating.
How much does it cost to repair a sill or quarter panel?
As mentioned earlier, this is difficult to say because every MX-5 is different in terms of the amount of rust. However, you can generally expect an amount between €500 and €1000 per side to repair this, excluding painting in color.
If there is only rust on the underside of the sills and the interior is largely intact, it will cost approximately €450-500 per side.
If the inner sill, part of the quarter panel, and possibly the wheel arch also need to be replaced, this amount can increase to €1000+ per side.
It is best to contact us and stop by so we can estimate how much rust is hidden behind the body panel.
If you have any questions about this message, you can always contact us by email or phone.