Probable because its hard the assess the condition of the underside with so much gunk. Ill bet you 20 quid it isnt a pretty sight under it all..
Smearing grease on the brake lines is an old one, my MOT tester even advised me to do it as they were looking a bit dry!
non of it works, neither soak in. They just trap moisture. Ive been spraying mine with a mixture of diesel and oil to slow things down until i have the time for my chassis overhaul.
I have a piece of machinery that i have to keep outside at the moment. Every couple of months i spray it with diesel. Been doing it a couple of years, works well and you can tell when in needs doing again by how the water drops settle on the surface.