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My old but reliable 2003 Ford Focus has just passed its MOT with no work needing doing, but the chap who did the MOT said the sub-frame is showing signs of corrosion. This is currently just cosmetic. The MOT report says:

Sub-frame corroded but not seriously weakened Front [5.3.3 (b)(i)]

Sub-frame corroded but not seriously weakened Rear [5.3.3 (b)(i)]

But the mechanic said that the corrosion is likely to become bad enough to fail within a year or two, and given the age of the car it would be beyond economic repair. The value of the car is probably only a few hundred pounds given its age and rather battered state.

Some decades ago when I was more of a car nut and drove a vintage MG I remember we used to regularly clean up the chassis with wire wool then coat it in waxoyl to inhibit corrosion. I'm wondering if this would be worth doing to extend the life of my Focus. But I'm not in a position to do this myself so I'd have to pay a mechanic to do it, and I guess that could easily come to more than the value of the car.

I realise it's impossible to make a definitive comment without seeing exactly where the sub-frame is corroded and how bad it is, but I would be interested to hear any opinions as to whether this is likely to prove worth while. I seem to recall that waxoyl doesn't last that long and I had to reapply it every year or so, but presumably you could just paint the sub-frame with Hammerite for a longer lasting fix.

Update

I'll add an update to this in case it's useful for anyone else with the same problem.

I took the car to my friendly local mechanic (not the garage that did the MOT) and he said the corrosion was very slight and that the MOT garage had exaggerated a bit. He did a quick clean with a wire brush and sprayed on some waxoyl, and he said that's all that is needed for now. His advice was to keep an eye on it and if it's significantly worse at the next MOT, or the one after that, then it would be worth getting aggressive with some POR 15. Until then just carry on.

If there is no structural damage, you should be able to take a wire brush or angle grinder with a wire brush attachment and clean the affected area, then using the Hammerite, paint the surface to prevent further damage. I'm assuming Hammerite is like our POR15 here in the States. (The Hammerite website really isn't clear on what it actually does, other than covers the surface.) If so, it should permanently bond with the metal and convert any rust to prevent further oxidation. Just be careful when you use it, because (at least POR15) will stick to everything and you will not be able to clean it off. Also plan on using the entire can, because once it's opened up, the little bit which slips into the lip seal of the can will permanently bond the lid to the can. You won't be able to reuse the paint brush, so don't go out and get expensive ones ... you just want to cover the surface and you really don't care if it's pretty (it's on the bottom side of the car!). Buy some throw away brushes and ensure you use nitrile (or similar throw away) gloves and clothes you don't mind getting stained ... you've got about a 99% chance of it happening!

Personally, I would clean as much of the exposed metal where you have the ability to do so and cover it with the stuff. The more underbelly you can get covered, the better off the car will be. While there may be slight damage, the fact it passed MOT is a sign it's not seriously weakened. Preventing further deterioration is the real key. As long as you can get it cleaned up and covered, you should be good going forward for a long time to come.