Rust and using a classic as a Daily Driver

wash the road grime off on a regular basis
under seal everywhere that is open to the elements / arches , under sills and i am forever just painting anything that will rust and cause problems
no real cure for rust probably inevitable but just keep on top of it plus a good few wax polishes helps , even used bees honey wax on alloys during the winter to keep the  corrosion at bay

I suspect there will be many diverse opinions on this one. Partly it depends how much work you want to do, if you're willing to go to town on the underside then stripping and epoxy will give you good protection. But if you're looking for something quick, cheap and simple then the best I've come across is the good old fashioned 50/50 mix of Waxoyl and 20W/50 engine oil. It won't last for ever but so long as you get underneath once a year and give it a fresh application it seems to work pretty well. It is a messy job though!

Waxoyl by itself, even when applied hot, tend to sit on the surface and doesn't creep into the metal - which is why you mix it 50/50 with engine oil. I've applied this to some quite crusty undersides and it has kept them going for years. The fact you need to apply it every year is really a bonus since it means you check the underside on a regular basis.

I have tried many expensive treatments and not found anything that lasts as well.

IMHO you will never truly stop the rust but you will certainly slow things down. Old car simply have more rust traps and the quality of the metal does not seem as good as it is now. I know my dad's '89 XR3i was welded on pretty much every panel underneath when he sold in in 2000. My Clio in comparison has never seen a welder and is an older car now. I don't think I have not owned a retro without it rusting despite applying underseal. This however seems to work for me.

1)Put cavity wax into all of the sections.
2) If you see flaking underseal do not leave it - it will trap water. Remove it until the underseal is good and reapply.
3)If you see surface dust deal with it ASAP
4)Keep the car clean. IMHO pressure washers are superb here. Any dirt that is hanging about in the wheelarches a pressure washer will remove it. I know with some classic cars when I have first washed them I have ended up removing what seems like a load of earth from underneath the wheelarches.
5)Water leaks inside the car will not help at all. Change the seals or leave them at your peril.

Oh, and use decent products that will not dry out. Bilt Hamber do great stuff here like their Dynax S50 and their underseal which is self healing.

It's one reason why I have stopped having a retro as a daily ; I simply got sick of welding in a new spot despite the underside being clean come MOT time. My MGB was welded on the chassis leg where previously it was fine. Same with the Stag. The last time I saw it the car had rotted out in a previously fine spot of the floorpan!

Get some plastic front wheel arch liners. Minis love to rot all around that area. I had some on a Clubman. The rest of the car was a crusty mess but behind the liners was like new.

I tend to hide from wet roads, and never go out in salt, but if the car does get grubby wash it immediately.

A good pressure washer is your friend here. I run a 10% TFR solution as a pre-wash and concentrate on under the arches and along the sills.

I would have agreed with you if I had the space to weld and fix the cars, not mention the time. When you only have a driveway for work and it is always being used by others, taking a few weeks to have your pride and joy welded up is not an option.

That and I am not rich enough to simply throw away the value of a car by neglecting it unless I buy a wreck in the first place, but then I cannot really fix them like I used to due to the aforementioned point and neighbours (they'd probably now have a field day if I was welding things up a weekend). Maybe you are .

Not sure about that, from a comercials point of view most of the Transits that have survived were the ones with the factory underseal option.

My experiance of wax oil type underbody waxes hasn't been too positive as its peeled off in sheets on one of my vehicles.

factory underseal is a different kettle of fish, mostly cos its being fitted to a brand new shell. I'm referring to cheap aftermarket stuff lashed on by over-enthusiastic 'restorers'. i see the underneath of a LOT of classic, vintage and retro cars and the general trend is the best condition ones have never seen underseal, whereas the worst ones have. just my observation though, no science behind it.

Firsy, pick a car that's had inside the sills rustproofed from new for an easy life

Put it up on ramps at the end of the summer (pick a warm dry day if possible),
wire-brush any loose stuff underneath,
weld up holes/scabby bits (hopefully none found)
*lots* of waxoyl (pour into soup-dish and apply with an old paintbrush)
long hot bath (you, not the car) to get rid of the rust flakes & waxoyl splashes.
pub!