Marc, true, we used to joke about the anti rust properties built into BMC cars.
There is a light spray of oil mist from every crack in the engine, like the felt seal on the crankshaft pulley, every cork gasket cover, out the rear main (no oil seal), sprayed from the flywheel out the gearbox adaptor, every cork gasket on the gear box, out the rear box seal and so on and so on. The under bodys wound up with a thick coat of oily dirt to add to the body "rotodip" underseal.
Floor pans never rust from outside, just from the inside because they never put any water drains anywhere!
The sills and rear tail section just eat away until the rust holes let the water out.
I drilled holes all over my underbody and ran sump oil through the chassis rails and sills, stopped the rust dead.
Modern cars just dont have this problem.
Fred.

I had an XC Falcon many years ago with rust starting to appear in all the usual places.
I removed all the door linings and sprayed fishoil liberally into the doors.
The car stank for months!!

One of the biggest causes of rust comes down to the panel design. If you examine the most notorious rustbuckets you;ll see two sheets of steel clamped together over large areas where they can collect dirt and water and stay permanently damp.

Compare the lower door edges to those of, say, a Toyota. There the panels are formed so that only the very edges are in contact and there is oil based sealer in the crimps. Better build quality also reduces the chances of water leaks into the cabin so the floor under the carpet stays dry.

HT-HG Holdens, XA-XC Falcons, Toyota Coronas and Datsun 120Ys were rust magnets because their designs allowed dirt build up in the car's cavities. They had holes punched to let the water out but when dirt got in over the years and blocked the holes, the car's fate was pretty much sealed.

With vintage Holdens, floor pan kits are available but they have to be welded in by people who know what they are doing or the car will buckle and doors will stop opening and closing, etc. If I ever fulfil a dream of owning an FC, if its floor needs a refurb, I will just send it to an expert who the clubs recommend and have it done by them. I can weld but it's not worth wrecking the rest of the car just to save a few bob.

Brad, that's right, the most ambitious thing I did was replace floors and sills in a Minor and by the time I finished I wish I had just gone and got a good donor car! You wind up welding to rust or trying to, unless you cut the panels way back to good steel. Then you have to form panels or cut them from a donor. I was young and stupid once. The body then looked like some amateur had got at it with wonky seams and endless grinding off and stuffing sealer all over it to hide the mess. It was solid but not pretty!
Too old to do something like that again.
Fred.

When you watch Youtube videos of some expert car restorers you soon realise what talent is required.
They hand make complex metal components to replace rusted out pieces and spend a great deal of time aligning everything.
When finished all the gaps between doors and guards etc are millimeter perfect.
I find them fascinating to watch as a rusted out hulk is rebuilt better than new.

I only did the one car in entirety.
While doing that I was fascinated to find out how much you can heat treat and beat and stretch sheet metal and make shapes.
I never got really good at it, I think the guys you see on those TV shows are on about their 50th resto and well versed in the practicalities of the craft.
I did weld up a set of headers about 10 years ago for my Morris from stock pipe pieces cut and weld to make a 3 into 2 into 1 and dump in the silencer. Popped in an oxygen sensor as well to drive the LED display
Made up the rest of the system with a stock truck muffler, biggest I could fit under the floor pan, got the exhaust shop to put the hump on a straight piece piece, that I could not do smoothly.
Some of the fabricating skills came back, but I wrapped the whole header system with asbestos tape to "keep the heat soak out of the engine bay". No, to hide the crappy joint welds!!!!
Now it sounds like a truck!!
Fred.

While doing that I was fascinated to find out how much you can heat treat and beat and stretch sheet metal and make shapes.

Steel is one thing. Watch this guy perform miracles on an aluminium fender: https://www.youtube.com/watch?v=9moYO-WWhaQ

On the farm mud is the enemy om the old cars, plastic does not rust. Normally if a vehicle comes in wet & carrying mud, the underbody gets the high pressure washer.

The idea of the leaky rear main is to lubricate the starter pinion. Zephyr & a couple of others have a weird exhaust system, which I have commented on before. All six cylinders individually vent straight into the exhaust pipe. That resulted in the iron pipe being flame hardened; It became brittle and cracked.

I got bored with that idea & since 1983 it has that pipe section (and the rest) made from 414 SS aircraft exhaust. Looks original until you pay attention.