What ever treatment you use - film, encapsulation, paint etc you still need to wash/rinse the underside. Nothing extravagant just a quick rinse. You can even set your lawn sprinkler on the underside for 10-15min and let it do its thing.....the neighbours might start wonder.
Over the years I have noticed that folks who use any of the above tend to believe/rely on their chosen method to be the be all and end all outside of yearly application. Most of the time the same people have a nice clean exterior and interior but the under hood and undercarriage are filthy.