I live in a place where the roads are salted and it leads to a lot of undercarriage corrosion. I typically get my vehicle undercoated every year (which costs about $150 where I am).
Even though I have undercoated it (typically via rust-check or krown) the frame still rusts (it's an old Jeep TJ).
My question is, is the undercoating doing anything beneficial if it's being sprayed on already rusted components? I'd rather not spend money if there's little to no value. And I wonder if it may actually be hurting things (by keeping moisture in).