Can you overspray the mentioned Bilt Hamber with anti gravel/ stone chips paints ?
End of argument. This member has been working on rusty cars all his life and knows the answer!
It’s Rustbuster Epoxy Mastic. Two coats and yer home and dry apparently.
No other opinions allowed: it’s the law!
He may be right but needs to sort out the presentation 
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Sadly , it was probably ice. I did a few stately exits from the saddle of a Norton Commando in that decade
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Yep, as you can see marketing over rules common sense (which is in short supply anyhow).
We live in a World where people think it’s a good idea to crawl around under a car every 6 months and spray expensive juice on there. Only for it to wash away so they can buy more and spray it on again.
Bonkers.
Ive not read many good reports on it online from users, i always buy Bilt Hamber personally.
I’ve got two aerosols (1500mls) of their S50 that come with 18” wands with a 360 spray. All for £35. Look just the job for the sills and internals of sub-frames.
They do the paint on stuff just like Waxoil et all but it always going to be eroded on exposed positions.
I might try the Epoxy Mastic suggested just to experiment. If is it flexible but how long does it last? My main concern is painting on over existing rust.
Under seal looks perfect- thick and flexible but it still has a finite life.
Whatever we use it’s not fit and forget.
Their UB is pretty durable but as you said although it contains inhibitors its not designed to be sprayed on bad rust.
Any rust should be cleaned back where possible and a removal type product used rather than a converter.
Ive used rustoleum combi colour paint under a car before and its a great product especially with the UB on top.
If it all fails at least you can buy replacement subframes for the price of an aftermarket exhaust /bright red braces to keep the wheels somewhere near where they are supposed to be 
As the owner of a ‘06 low mileage/low rust example I’m also wondering about the balance between cost and preservation. Driving around Cornish roads is simular to a maze with high hedges and blind bends. Done properly, the car is covered in ‘bramble rash’ on the OS. The chance of a slight encounter is quite high and you can get written off for a smashed headlight/wing these days. It a genuine dilemma.
Yes I try Lanoguard a few months ago.
The trick was just do a bit at a time. It was time consuming with all the wire brushing the rust & grime away but well worth it. The grease you applied becomes a bit of an art in the end as you get into the swing of it. The actual liquid wax (warm it up first) was prehaps the easiest part. Once again its spray and learn tactics. Too much and ends up on you and the drive.
Since I intend to keep my 2017 model a long time I think its wortwhile investment
Good advice…
I would say any underbody treatment DIY or having it done by a reputable outfit is 90% prep the application is then much easier from personal experience.