Repair rusted sills on a Mazda MX-5 NA and NB.

Rust at the sills and chassis beams (box girders) is a very common problem with the Mazda MX-5 NA and NB. Fortunately, there are repair panels and very rusty MX-5s are still salvageable!

Why does a Mazda MX-5 rust at the sills and side screens?

Rust forms when (untreated) steel is exposed to moisture for a long time. The sills and side shields are a “fine” example of this; the places where rust always appears on an MX-5 are the sills at the bottom and the wheel well edges.

These are areas where 2 layers of steel are spot welded together but there is still room for moisture. Once this moisture gets between these sheet metal parts, it has a hard time escaping, giving it every chance to act on the sheet metal until it is completely rusted away.

Therefore, in the photo above, you can see very well where the outer shell of the sill and the inner part are in direct contact. Thus, the occurrence of the rust is partly due to the construction/design of the sheet metal parts used by Mazda.

Recognizing incipient rust on a Mazda MX-5

Because rust always starts from the inside of a Mazda MX-5 due to the construction used, it is difficult to tell if rust is starting to form in the known places.

With the following tip, you can check yourself if your Mazda MX-5 is starting to suffer from rust at the sills and side screen:

Make sure your Mazda MX-5 is clean and then put it in an environment with plenty of light. Thoroughly inspect the wheel well edges and sill for uneven paint such as: small bubbles, bumps and odd spots where the paint is starting to come off.

If you find any of these then it is very likely that rust is present behind the sheet metal. The rusted sill/screen pictured above had only a few blisters in the paint….

Do you patch up or have the rust professionally repaired?

This is obviously a rhetorical question but I will explain it. With some regularity we receive Mazda MX-5s in the workshop that have been patched up before to pass the MOT or as a quick repair.

In the case of the sill pictured below, they left the rust in place, welded a repair panel over the rust and then finished everything tight. From the outside this looked very tight but as pictured below the rust continues from the inside.

Having it professionally repaired is the only correct way

In order to master the rust for many years to come, we at LucaCarMods always remove all rust that appears behind the sheet metal.

After drilling out the spot welds and grinding through the sheet metal, it is only really possible to assess the severity of the situation and which panels need to be replaced. Therefore, this makes giving exact price in advance impossible if we want to pass on a fair price to our customers.

How we proceed

1. Drill out spot welds and grind away the sheet metal to where the rust is visible from the outside. In fact, it may be that the rust is at an early stage and does not require the entire side shield to be addressed.
   

2. If the underlying rust is found to be higher up then we grind out more sheet metal to ensure that actually all the rust can be addressed.

3. If the underlying sheet metal is superficially rusted, we completely derust it and then treat it with a Zinc coating.
   
   However, if the sheet metal has become thin due to rust, the inner sill will have to be ground out and replaced with a new part that is also treated with a zinc coating.

4. Now that the inside is coated, we grind 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 there is a gap of about 1-2mm everywhere to weld.

5. When welding sheet metal, there should not be too much heat in the sheet metal. In fact, this can cause the sheet metal to pull, creating creases or bumps.
   
   We do this by moving up a few centimeters again after 1-2 short suture/point welds and doing 1-2 short welds again here. This ensures a better distribution of heat.
   
6. After welding, we sand the weld completely flat so that it is no longer visible, if at all -see photo below-.
   
   In terms of finishing, there are 2 options at LucaCarMods:
   
   1. We make the weld completely tight with a very thin coat of putty if necessary and finish the entire sill with a rough black crushed stone coating. As is standard on many 1st generation MX-5s.
   
   2. A zinc coating so that the sill remains free of rust and the customer can take the MX-5 to a sprayer who will finish the sill completely smooth and paint it in color.

7. Finally, we will always drill a small 10mm hole on the inside wheel well so that a long hose can be used to apply the cavity wax coating. We use Dinitrol 1000 Cavity Wax for this purpose. This ensures that beyond the zinc coating there is an additional protective layer applied in the sill.
   
   We seal this hole again with a plastic cap after coating.

What does it cost to repair a sill or side screen?

As stated earlier, this is hard to say because every MX-5 is different in terms of the amount of rust. Typically, however, you can expect to pay between €500 and €1000 per side to repair this excluding the paint job in color.

With only rust on the underside of the sills and where the interior is mostly in tact, it will cost about €450-500 per side.

Is the inner sill, part of the side shield and possibly the wheel well to be replaced then this amount can reach €1000+ per side.

It is best to contact us and stop by so we can estimate how much rust is hiding behind the sheet metal.

If you have any questions in response to this post, please feel free to contact us by email or phone as well.