When crawling under my sandblasted Mk1 body, I’ve noticed that the sills are open, as opposed to modern car design. Each has four broad openings along the edge under car, each about 2" (pic).
I wonder how shall I react to this fact … Does Mk2 also have open sills ? Was this idea dropped as faulty ?
Should these be plugged-up after doing inner wax protection through revision lids at the rear end ?!
Yes, this is right, same on my S-type 1967.
They have to stay open for ventilation.
Regards,
Peter Jan
Same on my MK2. I guess sooner or later water will get in so best have a way to let it out 
Dave
That sounds reasonable, however quite unlike today solutions. This design puts even more importance on sills’ rust protection from the inside.
Does anybody know what was the factory treatment for them ? I would prefer to refresh the coating with something similar, to avoid any cross- reaction. Bitumic ? Wax ? Or just a paint ?
The factory did not much, i have seen factory pictures of dipping a MK2 in some sort of primer.
That’s it, that why they rot 
I treated all hollow spaces with ML wax.
Regards,
Peter Jan
Truer words: if any closed pockets exist on a car, they will somehow, get water and dirt in them, and rust out. Better to be open and protected with some kind of protection like wax, and have the ability to be washed out.
When I repaired the sills on Tweety, I made mods to periodically power-wash the insides of then.
Okay, then what about side members ? I see a number of tiny holes on top and side (like for fuel line clips and others) but no real way in - to get a spray-on wax protection into it. 
On Tweety, I made 1-inch holes in the end of the sills, just ahead of the rear tires. That way, I could get a wash wand inside and flush them out. I also made sure the drain holes were open and not smashed shut: that is what caused my passenger-side outer sill to rust out.
Then I capped them with plastic plugs.
My car has 1 inch ( maybe 1.5?) in the ends of the sills as Paul suggests making. I thought they were original. In one of the pics at the top of the post Piotr it looks like there is a round plug/cap?
Like Paul I levered open the drain slots where they had been chrushed. It gave me enough room to get the wand of the Dynax s50 into the sills.
Dave
Sill rust protection seems clear, but not so with side members. They do have holes for unwanted water entry, but no factory drain / maintenance holes. Has anyone decided to drill any ? I would guess the area leaf spring mounts would be the way to go.
I used the holes for the fuel and brake line clips as well.
At the front of the chassis legs there are some holes. Not easy to reach but if you look down the back of the radiator and spot the brake lines you will see they are screwed to the top of the chassis leg. In the attached pic the screw is out which leaves a hole but you will spot behind a larger hole. This was on both sides of my car and I used these. Now I did it without the radiator in situ…
Cheers.
Dave.