Undercoatings, sprays, etc. are hit & miss at best. If you truly want to actively inhibit rust, even inner doorseams, lug nuts, hood hinges, et al., I wholeheartedly reccomend that you go to www.counteractrust.com. I live on the ocean & installed one of their systems on my last resto project, 1986 Turbo Saab: as well as, my 1993 Ford F250, without undercoating: It works! I have ordered on for my 1985 190E that I just brought over from Idaho. It is rust free & I will keep it that way. My 2 cents.
Richard