How severe is this rust?

[Conor D]

The MOT for my car expires in Decemember so I need to start making a move to make any repairs, I had a good look at the sills today and I took a few photographs of the undersides of the sills and also the sill under the Sill protector.. Hopefully I can get some opinions on it.

Left Hand Side:












Right Hand Side:













Also, Front right:




I took the car out to a body guy we've used before, and he reckons that the left hand side (sill) should be fine but the right hand side needs a bit cut out and welded in (which I don't doubt) - and he gave me a quote which is much cheaper than ones I've heard before.

I'm worried that it could possibly turn out to be worse than it appears on the inside, but I don't know if there are any signs of that. Are the traces of surface rust along the edge of the sill joining it anything to worry about, or the piece at the front right hand side?

[WarrenG]

The only way to satisfy your question is to take a screwdriver and chip away the rusty stuff until you find good metal, you could use a wire-brush wheel on a drill and try to find the bright edges. (that you need to weld onto)

It is likely you will find holes and guarantee that welding will have to take place, but at least you don't have to take anyone's word for it, you've seen it for yourself then.

As a comparison, the bubbles on the outer part of the sill at the rear are very similar to what mine had. And this needed a 6" sq patch on one side and a 4" sq one on the other - I got away lightly.

Unfortunately the build of these cars involved the spray-on stone-chip product for the sills, which forms a rubbery layer over the metal before painting. Very effective for corrosion prevention of the type that starts on the outside. Sadly though, the rot on a '5 sill starts on the inside and is held back by this rubbery layer, until it bubbles through. When you pick at those bubbles, expect to lose 2" square for every bubble.

[Evangelion]

As WarrenG said, these things rust from the inside. If it's this visible, I wouldn't be surprised if both sills have had it.

(Sorry to be the bringer of such tragic news.)

[MX-5 Lazza]

The underside of the sills rust from the insides. Most of that rust is just where water was getting trapped under the sill covers so is probably from the outside in so could probably be cleaned up ok. It's the rust under the sills that looks more serious.

[anonymous-user]

Interesting comments here, as my sills look in very similar condition to the OPs, perhaps better internally.

I too think the rust internally is probably from water pooling either under or adjacent to the stainless steel sill plates, probably dripping from the door drain holes?

Interesting comments on the rust running along the seam under the sills, mine has this and I though it perhaps just surface rust from water running down the outside of the car and sitting right along there as it's the lowest point, before dripping to the ground, or am I being too hopeful here?