Guys I gotta tell you I love my XJ but it's got a big problem. The roof is rusting out. At one point I tried to sand it down and prime it. It took me a whole day to do a small section and the rust came right back. What do I need to do to resolve this?

 

You can't just leave it primed, because as you found out it will rust right back. You need to top coat it with a good quality paint.

Also, sanding is not all you should be doing. Sanding is at the end when you want to feather out the paint for repaint. Wire brushing (preferably on an angle grinder or at least a fast drill) and something like this is also helpful:


Just be careful with the chemical method. In my opinion, if you use a good top coat, it's not all that necessary. Look for videos on methods, etc,. You can get it perfectly clean, and if you don't top coat it, it will rust again.

 

What I've done is:
Scuff off as much loose flake and scale as possible with a putty knife and wire brush.
Dry the embedded moisture out of the rusty metal by sponging it down with 100% isopropyl alcohol from the hardware store and letting it evaporate.
Hit it hard with Ospho or Rust Mort.
After the Ospho is done doing it's magic, dry it again with alcohol.
Hit it with two three coats of primer and five coats of paint two coats of clear.
30 days later wax the crap out of it and again every month for a year.

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that absolutely requires media blasting, priming and painting with 2 pack materials

anything else will means the rust comes back

lived all my life in a hot, but ocean side climate, seen roofs like that before

 

fluid film

 

As others have said it needs to be properly sanded, cleaned, primed, and painted. The good thing is the serious rust seems small now, so get at it before the bad spots get bigger.

 

thats a "last chance" roof

my mate was an el cheapo panel beater for over 40 years, we live right near the beach, he did hundreds like that

those rust bubbles near the front left would probably have penetrated. Every possible spec of Fe2O3 needs removing and replace it with...something else, preferably fresh new steel

then cover it with something non-porous (so not high solvent products)

 

Not all primers are water proof! Get a quality epoxy primer like DP. a media blast is best, but you got to go easy, too aggressive a blast will permanently warp, i.e. stretch the metal, so the blast operator has to blast differently than say if he is a blasting a thick cast iron part.

Then a good chemical prepaint wash is oft used, like a Rust Mort or simular. (phosphoric acid with zinc is typical) then a water proof epoxy primer but check with the maker to assure it is compatible with the wash coat, and top coat.

Major paint makers oft have helpful advice, call or write them. ppg, dupont, 3m, sem, etc..... they also have tons of good info in their tech sheets, often available online, read them!

for rust thru areas, well you now have to deal with both sides of the metal, fortunately your rust thru is likely only in a few spots. pull the interior trim to access. treat both sides as above. small holes can be filled with a water proof filler, even jb weld can work here for small holes, regular ole Bondo is not water proof! after the filler, then recoat with epoxy primer ( I run epoxy primer both under and above fillers)

it you dont address this properly very soon, the roof will be a lost cause, and replacement will exceed the worth of the vehicle in cost.


pay attention to cure times between coats, follow mixing instructions to the letter, get compatible products, for instance some primers will not cure in an acidic enviroment, so an acid wash maynot be advised under that primer. the paint makers will be able to tell you that. avoid eastwood coatings, they are garbage.

remember best bet to stop the rust is to get the roof media blasted, then coated properly with DP primer or equiv. if you skimp on the blasting or any other step, rust has a good chance of returning, then you get to do the whole thing again, or you got a convertible xj. Hire a shop that is experienced in media blasting body panels! Panels can be ruined fast with wrong blasting technique. you can spend days sanding, wire wheeling, and it wont be as effective as an hour of proper blasting. wire wheels also tend to microscopically smear good metal over rust, not good! also power sanders and wire wheels can like bad blasting technique, warp the panel, so much pressure, too high a speed and the panel,is damaged, warped.
of course blasting is messy, media gets every place, and glass, and painted panels must be protected, masking paper wont protect against media blasting. you will need heavy tarps and such to protect these items from stray media blasts

 

I can confirm that a layer of diet mountain dew with a little water will strip the rust right off. Leaves the metal bare and ready for paint. Tape off the roof with some plastic or something so you have a small "pool" for your diet dew. Put like half and inch of soda covering the area, leave it to soak for a few days, swirl the dew around once or twice a day. Just be sure to take your roof rack off and waterproof plug those rack holes so it doesn't seep into your car. I know it sounds like I'm lying about the diet dew, but it works like magic. Also, it doesn't attack paint.

 

That's the citric acid in the Dew doing what it does.