Hi,
My 2012 model year XF with 129,000 miles just got an advisory for rear subframe and chassis corrosion on the MOT, i was surprised and thought the tester was a bit overzealous but at the weekend i got it jacked up and had a look.

I got the wheel arch liners out and cleaned everything around the subframe, mounts, wheel arch (body side) and sill closures, after 5 hours with a soft wire brush, then jet wash and I was quite surprised at what i saw. The subframe is a very complex fabrication of pressing and welds and it only has a light coat of black paint, no other protection, the exposed areas and around the welds had have surface corrosion, some pitting and scaling but nothing that could weaken it. Inside the wheel arches on the body side the only protection was body paint and seam sealer, even on the end of the sills and a re-enforcing section underneath so the paint was lifting with surface corrosion underneath.

The next day i spent another 4 hours with a wire brush on a drill and a paint and corrosion removal wheel, on the subframe it was difficult to get to all areas which is why it took so long. Now i have coated it all with rust converter, next weekend will be zinc primer, stone chip / chassis paint, underbody wax and cavity wax in the sills.

I would seriously recommend getting your cars cleaned off underneath and sprayed with corrosion protection. My 20 year XK8 has had this done a couple of times and has no rust, my 1993 Mazda MX-6 has much better factory protection and is in much better condition than this XF.

See pictures below. I will post more next weekend.

After soft wire brush and jet wash, sill end and body mounting points for subframe. Off side

near side suspension mountings after light cleaning.

sub frame after heavy scrubbingl

Another view of the subframe after heavy scrubbing.

 

This does happens when snow is present in the area where you live and the road then sprayed with salt (sodium chloride & magnesium chloride) as an anti-icing measure. Or maybe you live in an area nearing the sea/ocean shore where salt is present.
What you did was excellent by applying (copied & pasted from OP) rust converter, next weekend will be zinc primer, stone chip / chassis paint, underbody wax and cavity wax in the sills.
Now, to prevent this from ever happening again even if you do not have snow but constant rain all year round, when spring has arrived, do a yearly underbody wash using a water hose or a water pressure if you have one and make sure the dirt, dust, mud and road grime that may have already accumulated that are stuck on the frame on the vehicle are gone. Run your fingers and a water hose to the spot where there are hardened mud or grime especially the underside & wheel mud guards have accumulated.

 

I have never actually seen a rusty XF and looked at many beat up cars on FB marketplace and Craigslist.

Did your car come from an area closer to the sea?

 

cut back the first layer of skin and you’re left with an s-type. of course it’s going to quietly rust out

 

To answer the question first. I’m not near the sea, I’m in the Midlands so as far as I can be in the UK. Also I keep the car clean and hose under the wheel arches when I wash it, I don’t service it myself though, if I did I would have seen it sooner.

anyway, it has now been rust cured, zinc primed, stone chip paint, under-sealed (non setting) and then also give a coat of underbody wax and I treated the doors, sills and other various cavities. Luckily I have all the gear for my classic cars.

this car won’t be rusting any more.

here are some more pics.

 

Move to Kansas.....or Spain...... Everything rusts in the UK😂😂😂

 

I live in Michigan. We have the worst corrosion in the country.

 

Because they salt the roads to melt the snow and ice, and the proportion of the year they have to do this is higher than others. Basically the same in the UK (where I spent the first 33 years of my life) but obviously on a much lower scale, so the corrosion is a lot lesser. Most of the time it's just raining or "too cold to snow" so snow/ice is not as common. French cars like Peugeot/Citroen/Renault were/are very popular in the UK, probably primarily because they're cheaper than German or Japanese cars, but I feel like one thing that may have contributed to the rise in popularity in the 1980s-90s was because the French started using galvanized steel long before the Japanese or British car makers did, and the effect this had on rust prevention was fairly significant, and it was not long before (especially) Japanese cars gained a reputation in the 1980s for rust, despite the fact that their mechanicals were incomparibly more reliable than (e.g.) British or French cars at the time. When it came to corrosion though, the Italians were King I'm pretty sure the Lancia Betas were already rusting by the time they reached the UK dealerships.

Now obviously all car manufacturers know about the various methods to prevent against corrosion but the level/duration of salt in the rust belt is brutal.

 

Here in Texas I don't have to worry about rust so much, but most of the cars I've restored came from rustier states or were neglected so I've seen a fair bit. But I like to watch several British shows about car restoration on television and it always shocks me the level of rot. Cars in the British Isles seem to rust in places that I'd never dream of. Also, I've noticed that most of the time when they are finished, they take their test drive in the rain. I guess if you live in England, you have to learn to accept driving your classic in the rain. Here in Texas, if it happens to be raining when we want to drive, we just wait an hour for the sun to come out and dry everything up again.

But rust preventatives have their downsides too. I once restored a car that had been "Ziebarted" and found zero rust, but I also swore to myself that I'd never do another Ziebarted car again. Ziebart always coats the engine bay too and that stuff was almost as much work to remove and refinish everything as doing rust repair on Car SOS. Of course the American car show tradition is to display with the hoods (bonnets) open so of course it all had to go. I can only imagine why the Ziebart guys are so overzealous with their application. Perhaps they'd watched British car restoration TV programs even 50-60 years ago and that's where they got the idea to spray all the way up the firewall and the underside of the hood too.

I applaud our OP for his initiative. We now know at least one X250 in GB will survive to become a classic. But keep all that stuff out of the engine bay please.

 

I like to keep the engine bay clean and shiny as well so I only used cavity wax around the engine bay and there are some rails going front to back quite low down so I coated the outside of those with cavity wax.

I once had a triumph dolomite sprint that was ziebarted round the engine bay. When I drove the car hard on hot days the coating would start to smoke.

it didn’t stop the front panel and rear wheel arches rusting away though.

 

Still, lovely job on the above. I'd definitely do the same if I needed, but fortunately this car came from Texas -> California (which brings its own problems in the form of faster hardening and eventual failing of plastics and rubbers).