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Just had my annual check done. I ALWAYS insist on a thorough check, and looking for what might go wrong. Also oil change and cooling fluid change and special rubber buffer check on the suspension. Wanted the brake fluid changed, but they said, it was 100% ok, so didnīt change it.
I alternate for the annual check beween a small shop, where I know everybody personally, and I tell them to THOROUGLY check all, and next year the official Jag service. Where I also tell them to look closely.
At collecting the car, I, as always, asked, whatīs on. They told me, everything fine. But they recommended to rust proof and conserve the under /inside of the chassis with wax. Some wax, they recommend, and use it on several cars. I should mention, that that special service shop REALLY does repairs and services oldtimer. Puch Gīs are a special obsession of the owner. They really awake corpses to live in Puch ( Mercedes ) G class! Also are really good on classic British cars, several RRīs, Bentleys, Jagīs on regular service there. Their daily bred are Mercedesses. Also elder ones, 600īs, AMGīs ...
My reply was, that my Jag has an Aluminium chassis. THEY said: everything?
And thatīs the question: are there any steel chassis parts to protect?
If in doubt, I would rather do the wax protection: itīs little money, and I LOVE that car!
Havent heard of waxing the underside before, but there certainly are steel parts under there, and they do rust. Seems like youd want to remove any rust and paint the affected areas first before waxing it.
aluminium oxidates. i feel like if they fully understood their own recommendation, they'd have pointed that out instead of just saying "everything?" and accepting the implication that because its aluminium the wax is completely useless -- but perhaps you left that out for brevity. anyway, not a criticism on either point!
the oxidation is shades of grey to chalky white rather than brown and red, but it is oxidation all the same. if you want to correct it, you might, if you want to inhibit it further, you might. just want to make sure you're aware. you can have a look for it and see whether its already thoroughly taken hold, maybe ask them if they can show you a before/after of an undercarriage they've previously done. that would help you gauge whether you really want this. i suspect it is labor intensive. seems like you'd have to wire wheel, sand, buff, wipe clean, and then heat with some kind of large oven coil and fans, the entire undercarriage. otherwise you're just sealing in the corrosion with some wax. anybody can lie on their back and smear wax all over that stuff.
I know people who spray their entire undercarriage with a mix of Bar & Chain Oil every autumn to avoid corrosion. Fifteen years later and the entire undersides of their cars are near perfect.
Iīll have to dig into this. I suspect they want to just coat the under - and inside of the underside thoroughly with a proven wax. I am sure there was no word about metal preparation beforehand, just spraying it on and inside of everything.
Locally we have a Krown rust protection facility. They spray an oil-based inhibitor on the entire undercarriage. My 2013 F150 has had it and looks almost showroom new underneath. It's an annual application.
I know people who spray their entire undercarriage with a mix of Bar & Chain Oil every autumn to avoid corrosion. Fifteen years later and the entire undersides of their cars are near perfect.
this makes a hell of a lot more sense to me because it's quicker and the oil permeates through rust easily. then again, i guess one could heat the wax up a lot, and spray that on. maybe it's just that simple.
I believe jons has hit the main concern. Unless you first eliminate all traces of corrosion, you're just encapsulating it. That might slow it down, but on the other hand, if its not completely encapsulated (which may be near impossible without significant disassembly) you actually concentrate it near any gaps in the coverage and speed it up there. And then its still deteriorating under that fresh coating, albeit maybe a little slower. Its generally only a good idea to apply some sort of undercoating when the car is brand new. And I still wouldn't do it.
My perspective tho comes from years of restoring classic cars as a hobby. Early on I made the mistake of taking on a project car that had been Ziebarted. Ever since, if I see undercoating on a classic car, I run, not walk away. No, I didn't find a lot of hidden rust, but rather our American tradition is to open our hoods at car shows and otherwise show off all the fine mechanical details of our marvelous machinery. Show cars are supposed to be as shiny and new underneath as they are on top. So I ventured to attempt to "remove" all of the Ziebart coating. What a massive job that was! There's not very many cars that you could actually restore and make your money back on but if you had to pay somebody to remove undercoating, that one job alone would easily cost several times more than your XK is worth and then you still have to repaint everything that was painted at the factory and replate all of the metal finishes that got damaged in removal.
I haven't restored any all-aluminum bodied cars so I don't have a sense of how bad they can get. I have dealt with a lot galvanic corrosion where aluminum rivets were used to assemble fiberglass bodied cars to steel frames and that can be horrendous so I understand the concern and desire to protect even aluminum-bodied cars. In the restoration world, after we're done, we just treat everything with penetrating oils. I'm not familiar with the Krown mentioned, but it appears to be something similar to my go-to, Gibbs Brand. The advantage to us restorers is that the judges don't deduct any points for a bare metal part being treated with penetrating oil like they would it were coated with something non-factory. The disadvantage to penetrating oils is that it tends to need re-applied after a few years (probably more often on a daily driver). But again the advantage is that there's not much you can do to your car that's easier to DIY, except maybe washing it. The hardest part is literally jacking it up and supporting it for access. I'm sure that Krown company charges a lot of money to treat a car in their facility, but you could buy a couple of cans, some old rags, disposable brushes, gloves and treat all the exposed metal underneath your car yourself in a couple of hours. And when/if you decide to sell it, you haven't scared off potential collectors/restorers with a bunch of ugly aftermarket coatings.
As I live on a beach within the salt spray area for almost 4 years I've been pretty worried about my jag. Per a recommendation on this very forum I purchased LPS 3 Premier Rust Inhibitor and have occasionally given a spray coat to all of the fasteners and steel parts I could see on the underside and under the hood. I did that maybe once per 6 mos the first few years. It is flammable while wet so make sure everything is good and cool. I haven't seen any signs of corrosion but i'll admit I haven't looked in over a year, but i've been parking a few hundred feet further from the beach during the past 6 mos. Unfortunately, I do have some corrosion bubbling up under the paint at 2 or 3 spots though. Just dime size spots for now. I really need to go to a paint/body shop and get it taken care of but I fear it will be a forever problem.
Could care less about washing or waxing under my car, but I like the looks of that for my toilet. That would get my more important undercarriage sparkling clean.
Impregnating underside with wax?
What a massive job that was! There's not very many cars that you could actually restore and make your money back on but if you had to pay somebody to remove undercoating, that one job alone would easily cost several times more than your XK is worth and then you still have to repaint everything that was painted at the factory and replate all of the metal finishes that got damaged in removal. 
