For a NEW vehicle you plan to keep a long time which is very clean underneath, I would use a traditional rust proofer like Ziebart but would not go back for the periodic re-applications. I believe the re-applications do more harm than good.
My experience is oil sprays will wash off the exposed surfaces, which is why it needs to be reapplied. Vehicles that get sprayed several years in a row for the first few years (like my father in law's work vans) can then get neglected for multiple years after the last undercoating before it starts to inevitably rust out where the product has long ago washed away.
I pulled the sill plates on my last 2 trucks and sprayed this area with fluid film. I have worked on a lot of trucks with similar corrosion from the snow and salt mess that comes from living in the North. On my personal vehicles I use good mats and will often throw towels down to soak up the water if I am in and out of the truck a lot. I have seen a couple that were rusty like that because the trucks were repainted when new (service vehicles) and the door gaskets were damaged or not reapplied correctly and leaked down the "A" post and into the trench.
That's why I suggested Ziebart. They don't spray oil on it. It's like a black tar like coating.
You should watch South Main Auto channel, he wont even work on cars that have ziebart applied and he shows cars that have been ziebarted and and how it hides the rust damage and causes more harm than good.
What the S.Main auto channel thinks is meaningless and that is why I would never go back for re-applications. Those applications do more harm than good.
How did your Ziebart coating cure? Is it still soft or has it hardened up.
One of my old Novas had been Ziebarted back in the 70s with their old-formula black tar stuff. It had hardened and would either a) flake off revealing the horribly rusted steel behind it, or b) the steel would rust away completely leaving just the tar substance remaining. It was totally worthless as a rust preventative.
