Yeah drop the arch liners, clean it all out and then spray some waxoyl type stuff, but not actual waxoyl as it's crap. I use Würth underbody protection wax. My dealer mate gave me a load of cans years ago as it's what VW use at the factory, or used to at least.
It's more like a thin 'fluid film' and I cake everything underneath with it, especially the rear trailing arms and subframe etc which rust up pretty badly.
You can get thicker, gunkier products from POR15 or Dinitrol if you want high end stuff!