Fluid Film Review + Fluid Film, Krown, & NH Oil Compared

The underbody of a car is shown as it's on a lift; rust apparent

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Automotive undercoating is important to help prevent rust and corrosion, especially in colder climates with roads that are salted in the winter.

Fluid Film is a popular and well-reviewed product, as are its biggest competitors, Krown and NH Oil Undercoating.

In general, the DIY options offered by Fluid Film and NH Oil Undercoating will be the cheapest options, but a professional application (offered by Krown and NH Oil Undercoating) will likely come with a warranty for future maintenance.

For more about Fluid Film and its competitors, see below.

What Is Undercoating?

Undercoating is applied to the metal surfaces of your car (primarily the underbody) to prevent water and salt from sticking to the metal and causing rust. After application, oil-based undercoatings displace any moisture that was clinging to the car — without water, rust can’t form. The oil spreads into all of the nooks and crannies of the vehicle, protecting places that can’t be seen. Some undercoating products can also be sprayed inside the doors to prevent rust and lubricate things like the window mechanisms.

Oil coating products are different from a “traditional” undercoating offered by a dealership on new cars. Dealerships will often apply paint or wax to the underbody, but this type of undercoating is typically not an effective way of preventing rust in the long-term. Over time, the paint may start flaking off, or the wax may start to crack, allowing moisture to penetrate the metal surfaces and begin corroding them.

Oil-based undercoating is proven to work especially well in regions with harsh winters, where cities apply salt to the roads for safety, as salt increases the speed at which metal will rust.[1] Of course, for an oil-based coating to work, proper application and reapplication practices must be followed. For example, auto experts recommend that the undercoating should be applied every year, typically in late summer or early fall.[2] Applying a protective coating once, such as when you purchase a car, will only protect it for that year.

Does Fluid Film Work?

Fluid Film is an oil-based automotive undercoating. The product is non-toxic — its base ingredient is lanolin — and it contains no heavy metals, phenols, arsenic, or other harmful ingredients. The product is available in spray cans or larger-quantity undercoating kits, and you can apply it to the exposed metal surfaces or underbody of your vehicle yourself.

Reviews & Effectiveness

As noted by users of the brand and Fluid Film’s FAQ, the product will not remove existing rust, but it does stop rust from spreading and prevent future corrosion if you keep up with the yearly applications.[3] Fluid Film is an effective undercoating, according to both mechanics and customers. Reviews state that the product typically holds up well in snowy winters with regular reapplication, and it penetrates into most parts of a car’s undercarriage.[4] These reviews corroborate the company’s claims about the product. Stotts Industrial, the manufacturer and distributor of Fluid Film, also provides additional customer testimonials.

Benefits of DIY Undercoating

While dealerships and service centers may charge high prices for undercoating services, you can apply Fluid Film yourself, making it a cheaper, effective way to protect your car from weather and the elements. Fluid Film is widely available at auto parts stores and other retailers, including websites like Amazon and farm equipment retailers like John Deere.

DIY undercoating can be messy and requires some preparation, such as sanding existing rust and washing the vehicle’s underbody,[5] but it may be worth the effort if there isn’t a professional undercoating service near you, or if the professional services near you are costly.

Fluid Film’s aerosol spray option also makes touch-ups quicker and easier to apply than some other do-it-yourself undercoating products. Occasional touch-ups throughout the year are recommended, particularly on areas that are not protected from “road spray” (i.e., debris kicked up by the vehicle’s tires).

What We Recommend

In general, Fluid Film has the most cost-effective and accessible products for DIY undercoating applications. It is also the most popular and highly reviewed brand. However, if you’re looking for a professional application, both Krown and NH Oil offer the service at their service locations, and they both offer warranties for the undercoating (a benefit that Fluid Film does not offer).

Comparison Table

You can use the table below for a quick comparison of each brand, then select any product or scroll for more information.

Undercoating Brand Application Options Available Products Cost Warranty Options
Fluid FilmDIY Aerosol cans, gallons, or full kits $10 per spray can, $50 per gallon, $200 per five-gallon pail, or about $150 for a kit One-year shelf life for aeorsol products
KrownProfessional Professional application or rust protection & lubricant aerosol cans $10 per spray can; $130 to $150 for application Ongoing coverage with yearly reapplications
NH Oil UndercoatingDIY or professional Professional application, touch-up cans, or full kits $135 to $300 for a kit; $30 for touch-up cans Offered through participating dealers
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Fluid Film vs. Krown vs. NH Oil Undercoating

Fluid Film and other popular undercoating products — such as Krown and NH Oil Undercoating — are an effective way to prevent or delay rust and corrosion. Each of these companies offers different options for application, warranty coverage, and price points. The brand that will best fit your needs ultimately depends on where you live, whether you want to do the work yourself or have a professional application, and your budget.

Fluid Film

  • Application: DIY. Applying Fluid Film yourself is a pretty straightforward process. (See the video tutorial below.) Since the product contains no solvents, it is safe for use on most paint, rubber, electrical connections, and metal.
  • Available products:
  • Warranty: No warranty is offered for your vehicle, but all Fluid Film aerosol products come with a one-year shelf life.

The following video provides step-by-step instructions for Fluid Film application:


  • Application: Professional. A technician will put the car on a lift and spray the underside with the oil treatment. You may also choose to have your engine bay treated, as well as the inside of your door panels; both areas are exceptionally prone to rust.
  • Available products:
    • Professional application for a two- or four-door car — Around $130
    • Professional application for a larger car or truck — Around $150
    • Rust Protection & Lubricant aerosol cans — Around $10 each (not available on Amazon)
  • Warranty: Krown offers warranties for new and used vehicles. Both warranties require that you apply Krown annually and keep a record of the yearly applications. The warranties include body panel rust protection, with damage repaired or replaced at no charge up to the current Red Book value of the vehicle. To qualify, new vehicles must have been purchased less than six months before applying Krown.[6] The used vehicle warranty is good for a maximum of 60 months (with unlimited mileage).[7]
  • Find a Krown service location

NH Oil Undercoating

  • Application: DIY or professional
  • Available products:
    • Touch-up cans — About $30 each for black and clear coats (not available on Amazon)
    • Full undercoating kit — About $135 to $300 directly from NH Oil
    • Professional application — About $150 to $200 or more, depending on the location
    • One-gallon and five-gallon containers of undercoating are available in black or clear-coat colors. The undercoating kit includes the amount of undercoating you choose, plus a sprayer and wand.
  • Warranty: Participating dealers offer a nationwide, transferable guarantee for the life of the vehicle, as long as the undercoating is applied annually.
  • Find an NH Oil service location

For more on protective modifications, see our article about the cost of a clear car paint protection film.

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