there are many inaccessible areas underfloor and when the paint is dry, i will use some special penetrating paint and grease to protect the cavities againt corrosion.
Many years ago in Detroit I learned this trick.
Mix STP with kerosene and using a small flexible tube, inject it into all the areas you can't easily reach. The kerosene will evaporate leaving a thin film of STP.
I'm not sure where you would find it, but "rustproofing" places use special equipment for doing this. It involves drilling small holes to access places like door pillars and then closing out the holes with plastic plugs.
May not be the way to go if you are a purist but if it prevents rust in places like the "rust belt" of the US it's worth considering.
