Best underbody rust protection spray for cars depends on type you prefer, oil based creeping versus rubberized hard coating, but top rated products include Fluid Film, Krown, NH Oil Undercoating, Woolwax, and 3M Rubberized Undercoating. For annual oil based protection that is best for old cars and rust belt because it creeps into seams and does not trap moisture, Fluid Film and Krown are highly regarded, they are lanolin based, creep well, displace water, and need yearly reapplication, cost 120 to 200 dollars professional. Woolwax is similar lanolin based but thicker and longer lasting. NH Oil is also popular wax based. For hard rubberized coating that provides physical barrier against stone chips and lasts longer 2 to 5 years, 3M Professional Grade Rubberized Undercoating, Rust-Oleum Rubberized Undercoating, and POR-15 are top rated. POR-15 is very hard and durable for restoration. For cavity wax for inside panels, Waxoyl and Dinitrol are best. Many experts recommend oil based spray for existing rust prevention in salty climates because it does not seal in moisture, while rubberized is best for new cars with clean underbody. So best overall for rust prevention in salty winter is Fluid Film or Krown annual oil spray, best for long term hard protection on new car is 3M rubberized or polyurethane coating.
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