Repair Advise: Side Sills

[nintenjo]

So my 05 (50k miles) is starting to show a few small spots of rust on the rear arch but more so on the rear quarters at then ends of the sills. I am planning to get the bodywork repaired and sills replaced but I’m interested I learning why RX8 suffers with this issue? Are there any specific design flaws with the sills?

(I’ve owned a few mx5s in my time but the rusty sills were usually caused by certain design flaws like the rain rails plus it’s a much older car but I wouldn’t expect a more modern car like the 8 to suffer like they seem too).

I’m also asking so I can advice the body shop on preventative action to prevent it returning in future post repair. As I’d rather invest in having the car long term.


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[PaulAV]

From my understanding there are 2 issues firstly the complex sections are a trap for any water that penetrates into the cavity and secondly the roads in Japan aren't salted in the winter(I believe) so the Japanese don't get rust proofing right.

[nintenjo]

Yeah I understand your first point but I’m pretty sure the second point there is wrong? Mazda is a pretty clever company and know that rust proofing is important outside Japan? That same excuse was often touted on mx5 forum which while could be true for Japanese import cars wasn’t true for UK models.

Mazda 3’s from same era as 8 (04-09) don’t suffer from sill corrosion like 8’s do?


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[warpc0il]

This...
viewtopic.php?p=1271183#p1271183

Plus...
The structure of the Mazda 3 was designed by Ford, as it's just a Focus in drag, like a Volvo S40, which means it was designed not to have rust traps.

[nintenjo]

The side sills, especially from the rear arches, are barely protected from the elements, except by the (cloth) arch liners. Those arch liners themselves wick water into the metal they're held onto via the plastic clips that hold them on and once that starts to rust out, It's all too easy for water and dirt/leaves to find their way behind them and into the 'tunnel' of the sill. Once it does there's no way for it to escape. Given time this rots them out.

There's zero rust protection inside the sills, as I can only presume it was deemed a 'hidden' space that didn't require it.

There are a couple of ways I can think of prevent it from happening again once repaired, the first being to seal off the sill entirely. This may not last however and counterintuitively may cause the problem to be as bad if not worse than before, as water may find its way in via other means and be unable to escape. The second is to fill it with waxoyl or rust prevention resin of some description to act as a repellent/displacement for the water. I think true protection lies with a combination of rust preventative and providing sufficient airflow to remove any water that may end up in the sills.

As for Mazda being a bit enough company to rust proof their cars properly, yes. You'd think so wouldn't you? But a quick glance through the reams of rotting MX-5's and 8's out there shows this to be a fallacy. I have a feeling that their partnership with Ford for their 'conventional' range meant strong influence from engineers used to European and American roads where salt is liberally spread in the winter and so they designed accordingly. The same can't be said for their 'speciality' ranges.