Whatever the problem there is an expensive product for it.
But you don't need it, or the hard horrible work that goes with it - breathing in all the muck and bits dropping in your eyes under the van.
Just brush the thick loose muck off and brush some cheap thick gear oil on every year or so. Thats all you need to do. Easy
Years ago I was MOT testing cars in the days when they rusted much faster than they do now. But they also tended to leak oil - I often found rust beneath underseal and paint, but never beneath oil because the protective film self-heals.
But I don't recommend used engine oil because it stinks, and leaves black stains where it inevitably drips. Gear oil is thicker and sticks to the metal better. Clean oil washes out easily with modern detergents. Its the muck in used engine oil that stains your clothes and drive.
I bought a new car 30 years ago, and its still i regular use now