Preventing Chassis Rust

Having recently purchased my 2003 Discovery 2, I now like it very much and want to protect the chassis from rust. It is not bad but there is a lot of surface rust - I took it to have its fluids changed so that I could draw a line under this aspect of the service history and the mechanic described it as "a bit flaky underneath".
From my limited knowledge of repairing bodywork over the years, I am led to believe that to paint over the rust will make the problem worse, and I have therefore been skeptical about those underbody sealants. Am I correct?
What I'd like to do is strip it down part by part, clean off all the surface rust and cover with a good rust-preventative paint, but I do not have the tools nor ability yet to fully dismantle parts of the underneath.
So, are their any alternatives? Is it a good idea to wire brush the chassis and paint the underneath in-situ, or is this no good? Are those sealant treatments a good idea on rusty metal?
Thanks,
Andrew

High ANR and welcome to the forum.
Sadly D2 chassis, especially at the rear either side of the fuel tank, rust for a pastime. "A bit flaky" probably means you will have to do some serious patching if not a 1/3 or 1/2 chassis replacement sometime soon.
They all do it, shocking design fault and not soemthing you expect from a LR product that in LR terms is not that old. The good news is the bodies don't rust anything like the D1s do.
Many threads on this and your original question on here.
I put up almost the identical thread up to yours after I recently had my chassis extensively patched.
Here it is
https://www.landyzone.co.uk/land-rover/best-anti-rust-treatment-for-a-patched-d2-chassis.356352/
Best of luck!

Thanks,
I've just read that previous post. It's very informative.
Andrew

There is a guy near here who jet blast washes the underneath of cars down to bare metal ready for treating. He does say though, that if there isn't much left of your chassis after he has finished, don't blame him. I've never been brave enough to use his services, sometimes, ignorance is bliss.

Col

Wire brush then a good quality converter like Fertan, then Bilt Hamber Dynax should give you some time before it gets MOT fail worthy. But hard to say without seeing it.
Bilt Hamber stuff is really good a easy to do yourself. Dont get too freaked out if the rear is like swiss cheese, its all fixable.

I did on mine the last 3rd of the chassis, can be done quite well in situ. The only thing, and it is very important, remove the fuel tank as it can be the worst area. In places where it almost touches the chassis mud can remain there which keeps moisture=rust.
Also good idea to remove the wheels and rear suspension (air bag and shocks). After I wire brushed (and even used chisel or flat screwdriver to remove rust which is like a "layer") cleaned, passivated and painted.

I've just done the job using two tins (for brush application) and one spray canister of Bilt Hamber UB. I did the full chassis. The 'flakiness' was mostly old underseal and though there was surface rust, most of the chassis wasn't bad. Fuel use should be a lot better as I must have excavated a 12" pot full of dirt from cavities at the back
What an awful job though! I've ruined my best pair of overalls, and my favourite hat, and I was covered in rust and dust and UB spray. The Bilt Hamber looks good stuff though. Hopefully it won't need doing again for a few years.
Thanks for all the advice.