Stumbled upon this on one of the Facebook Subaru Outback forums. Someone posted about how the side skirts really have no drain holes, and if removed you will find all kinds of dust, muck, and old salt residue.
Living in the Chicagoland area, in the winter we go through a astronomical amount of salt. That is bad news for our vehicles, because, of rust. The side skirts in the picture below have almost no drainage holes underneath.
Underneath the side skirt it's all painted metal. That paint is really not going to protect or prevent your vehicle from rust.
After cleaning the all of the dirt, soot, muck, salt, and debris in the side skirt and body, I just used painters tape to protect the areas that I did not want to spray with the Rust-Oleum undercoating.
I sprayed on 2 layers on the side and underneath where the side skirts cover. I just made my vehicle better prepared for the upcoming winter and beyond. I was very pleased with the outcome.