How to spray underseal ( Photos)
 

In the UK we use something called Waxoil been about for 60 years , 5 litre is about £30.
When you get it its too thick to get out the tin so you need to put it in a bucket of boiling water and stir until its liquid. Pour off about 20% and top up with white spirit or petrol. Mix thoroughly and pour off into 6 UK milk bottles . Its easier to warm one bottle for a job than the whole tin.
You will need a spray gun and an air compressor 1.5HP is fine .
Now you can buy cartridges of waxoil for the gun but its cheaper to buy a tin.
Drill the top of one of the milk bottles (1 litre) and fit to the top of the pick up tube and hold with a small worm clip. You can then screw the bottle onto the gun . No bleed hole required the gun will suck the bottle flat. Re warm an pour off any surplus
Photos below

https://cimg1.ibsrv.net/gimg/www.jag...e973c017a7.jpg
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https://cimg8.ibsrv.net/gimg/www.jag...9bf988bfe2.jpg
The bottle on the right has been sucked flat.
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https://cimg1.ibsrv.net/gimg/www.jag...0ef6aff235.jpg
about £10
You can spray into the sills by removing the bungs front and rear and take off the tread plat and spray through the many holes it reveals

How often do you have to reapply?

I dont ..the idea never occured to me. You can get your car done professionally with Waxoil for about £600 and they give a 5 year warantee ....go figure

I believe this is an old school solution. Will this penetrate at all? Or just seal the rust and hope for the best?

There are two things here ...how you do it and what you do it with.
the idea of using disposable milk bottles is innovative whatever you spray.
What you spray is up to you and depends on what you can get hold of.
It certainly prevents the MOT test man seeing too much .
For dry scale rust then marine epoxy which takes 24 hrs to set and therefore has time to soak in is best but of course in a thin layer would be brittle.
As far as the UK is concerned Waxoil is the way to go .
You could of course put the body at 500v to disrupt the rusting process like they do on an oil rig. One of the reasons for changing to negative earth was that the positive earth cars rusted faster .

I havent seen a single milk bottle in Norway in my 55 years of life :)
Glass bottles was used until 1958, since then it has been waxed paper boxes! Strange?

Clearly these are plastic ..used by all UK supermarkets . but there are bottles with other stuff in with adaptable tops and you could cut or extend the pick up pipe.
Glass bottles are still used by some small dairies who offer a delivery service. Suitable bottles in France with UHT milk.
I note in US you can get Waxoil but only in very expensive aerosol cans.
Maybe you have some lanoline stuff.

That's awesome. I hate rust. TM