Reviews

Best Undercoating: Keep Rust from Eating Away Your Car

Our top picks for the best undercoats with the best rust preventive formula.

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BY Noelle Talmon / LAST UPDATED ON April 29, 2021

OUR QUICKLIST

Rust is the enemy for car owners because it can destroy the metal. Over time, pieces of your vehicle will start falling apart. That’s why it’s essential to keep rust in check. One way to do so is by applying an undercoating to the underbody of your car or truck. Undercoatings are designed to protect the bottom of your vehicle from salt, rock, gravel, moisture, and other corrosive substances. You can use these products to even out the surface area or as rust converters. There are many types of undercoatings available online and in stores, so it can be tough to choose the right one for your needs. We feature some of the best options in the buying guide below.

 

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Our reviews are driven by a combination of hands-on testing, expert input, “wisdom of the crowd” assessments from actual buyers, and our own expertise. We always aim to offer genuine, accurate guides to help you find the best picks.

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Benefits of Undercoating

  • Save time and money. If you pay a professional to put an undercoating on your vehicle, it can be costly. Plus, you have to make an appointment, set time aside to bring your car to the shop and wait for the results. If you do the job yourself, you'll save a lot of money, and you can choose when and where you want to do the project.
  • Minimize road noise. Over time, your vehicle's undercarriage can rattle a bit due to age and rust-related calamities. Undercoat products are designed to fill in the small spaces and seams beneath your vehicle that can be noisy when you're driving.
  • Provide additional protection. The best undercoat formulas will provide an extra layer of protection against dust, moisture, and corrosion. You have to worry less about gravel and other elements that may harm the undercarriage.

Types of Vehicle Undercoating

Rubberized Undercoating

Rubberized undercoating is easy to apply, and generally offers the best protective coating for your car’s undercarriage. It cushions surfaces against moisture, rust, dings, and dents. Rubberized undercoating is safe to use on quarter panels and wheel wells and dries with a soft rubbery finish.

Asphalt-Based Undercoating

Asphalt-based undercoating offers a higher level of protection suitable for large trucks, recreational vehicles, and heavy machinery. It’s formulated with some bits of rubberized material to aid in noise reduction. It offers extra protection against chips and moisture but is not paintable and, therefore, only suitable for undercarriages.

Polyurethane Undercoating

Polyurethane coating is easy to apply and works by sealing up cracks and seams. The compound displaces salt and moisture and protects the unoxidized metal from rust. It’s often sold in canisters that are ready to preload into an application gun. It, however, requires some surface prep work before application, like degreasing, and applying an etching primer to the bare metal.

Wax or Paraffin-Based Undercoating

Wax or paraffin-based undercoating sprays are the cheapest and quickest options available. However, their results don’t last long, and you may have to apply a fresh coat after only a few months. In addition, the wax isn’t very adhesive on a metal surface. Its main advantage is that it seeps quickly into cracks and seams, and offers moderate protective properties against moisture. 

Top Brands

3M

3M is a multinational mining and manufacturing company that produces consumer products for the healthcare industry, energy industry, and automotive world. The company was founded in 1902 and is headquartered in Minnesota. Some of its top products are the Dynatron 544 Dyna-Pro and the 3M 03584 Professional Grade Rubberized Undercoat.

Penray

Penray is a global manufacturer of chemical products for marine equipment and the automotive industry. The company has been in existence for over 60 years, and its products are engineered to keep your vehicle operating at its optimal performance level. One of its best-selling products is the Penray 4424 Rubberized Undercoat.

Rust-Oleum

Rust-Oleum is an Illinois-based manufacturer of protective coatings and paints for home and industrial use. Robert Fergusson founded the company in 1921, with the aim of manufacturing products that reduced corrosion on metal decks. The company has a number of brands under its umbrella, including NeverWet, NanoShield, American Accents, and Seal Krete, among others. The Rust-Oleum Rubberized Undercoating is one of its top-rated car undercoating products.

Best Undercoating Pricing

  • Under $30: At this price range, you will find a number of high-quality products that offer the best protection to the underside of your vehicle. For undercoating agents, a cheaper product doesn’t mean that it’s low quality. It may be cheap due to its packaging or volume.  
  • $30 to $50: This mid-range price level is for undercoats that come with an underbody spray applicator and may be sold in packs of four or less. Most products here are packaged in bottles of 11 to 32 ounces.
  • $50 and up: This higher price range is for top brand undercoats that are formulated to provide all-weather protection for your vehicle and offer maximum protection against rust. Most products here are packaged in 1-gallon bottles.
An undercarriage damaged by rust.
gettyimages

Undercoating sprays prevent rust from damaging your undercarriage. 

Key Features

Volume

Vehicle undercoating products come in volumes from 16 ounces to about 5 gallons. Think of how much undercoat you will use in the current job, the size of your vehicle, and how much you will need to buy in the near future. A 16-ounce bottle is suitable for a small car, but an RV or a truck will need a 1-gallon jug to get the job done.

Shelf Life

Most premium undercoating materials have a shelf life of at least one year after being opened. Others can only last for a few months, then they start to wear off. If you find a fully functional product with a short shelf life, it’s prudent to buy it in small portions. Figuring that you will use the undercoat at least once a year, the shelf life shouldn’t be an issue if you go for the right portions.

Formulation

The formulation you choose depends on your budget and how long you want the coat to last. Rubberized and asphalt-based undercoats are more expensive, but a single coating job can last up to a year or longer depending on the environmental conditions. Cheaper options like paraffin-based formulations last for only a few months, and you may not be able to stand the smell.

Other Considerations

  • Functionality. The undercoating spray should be able to at least prevent rust and corrosion on the metal surface. It should also be paintable, have sound deadening properties, have quick drying time, and improve the aesthetics of the sprayed surface.
  • Filling Power. The filling power is a measure of the undercoat’s ability to seep through cracks and holes, to smoothen out irregular surfaces. An even surface sets a great base for the topcoat paint and makes the vehicle's body look more appealing.

Best Undercoating Reviews & Recommendations 2021

Best Overall
Dynatron 544 Dyna-Pro

The Dynatron 544 Dyna-Pro is one of the best undercoats for rust-proofing a car’s underbody. It offers all-weather protection and can last through an entire winter season. The coat leaves a smooth feel, even on a rusted surface, and has a glossy look to it. The spray is powered by 3M, meaning that its molecules are tightly bonded with an electro-mechanism to form a strong barrier against rust and chipping.  


The product comes with an easy brush and sprays and forms a fast-drying coating. You will have an easy time painting on the coating with a standard refinish paint system, and it will match any desired OEM color. Moreover, it has great sealing, anti-abrasion, and anti-corrosion properties. It also deadens the noise from the undercarriage.


Its main drawback is that the undercoat doesn’t have great adhesion on greasy surfaces or an older undercoat. You would have to rub them off before you apply the spray, and that can be time-consuming. It also has a short shelf life once opened and is more expensive than other rubberized undercoats.

Best Value
Penray 4424 Rubberized Undercoat

The Penray 4424 is a budget-friendly undercoat that can be painted over with no bleed-through. It’s also a waterproof formulation that forms a protective layer against dust, road salt, corrosion, and extreme weather conditions. Penray comes in an easy-to-use aerosol can for easy application, with no drips or clumps. Its rubberized formula acts as an insulator to minimize road noise.


The spray is designed to seal, protect, and rustproof metal surfaces. The undercoat blends well with color paint finishes and even with a matte black finish. The spray can be used on all automotive applications such as the frame, wheel wells, and undercarriage. It also works on home foundations and for minor repairs on roofs and gutters.


Its disadvantage is that it takes too long to dry, especially when applied on a vehicle's underbody. You may have to wait for at least two days for the undercarriage to dry. Moreover, despite being a cheap product, you will need to apply multiple coats, and you may run out of the 16-ounce bottle before you get the job done. However, this spray is suitable for cars, trailers, trucks, and RVs.

Most Versatile
3M 03584 Professional Grade Rubberized Undercoating

The 3M 03584 makes our list as a rubberized spray that can effectively remove any traces of corrosion, abrasion, and dust from a vehicle’s undercarriage. It’s also an asphalt-based spray with a no-run formula for easy application, meaning you won’t have to clean drips from your garage floor as you apply the undercoat. In addition, like a rubberized spray, it will form a durable barrier against moisture, grime, and grit.


The spray provides insulation to a vehicle's cabin to reduce the noise coming from the undercarriage as it gets older. The coating also seals up small cracks and holes in an old undercarriage to even out the surface. You can also use the coating on wheel wells, trunk lids, quarter panels, under the hood, and the interior side of the fenders.


Its downside is that this undercarriage spray has an unpleasant smell when wet. You’ll also need to apply multiple coats to get a proper seal on the undercarriage and other metal surfaces. It’s also a newer undercoat in the industry and unfamiliar to many people. Despite that, the spray is suitable for trucks, RVs, and vans.

Most Durable
Fluid Film Rust & Corrosion Protection

This non-toxic, rust-preventive lubricant is a durable and thixotropic liquid that's used in the aerospace, aircraft, and automobile segments. It's solvent-free, won't dry out, and will penetrate to all metal bases. It provides corrosion protection and salvages significantly corroded and frozen parts, including nuts and bolts.


Overall, this is an excellent rust preventive coating for cars and trucks. It creeps into all the corners and crevices on your vehicle's underbody for optimal coverage. It is also easy to apply, is all-natural, and repels water, snow, and ice while retarding rust.


However, the product is gooey and has a strong odor. During the first few drives, it may produce some white smoke.

Honorable Mention
Rust-Oleum Professional Grade Rubberized Undercoating Spray

This rubberized undercoating spray is designed for bare or primed metal underbodies on automobiles, trucks, trailers, and RVs. It protects against corrosion as well as water, salt, and chemical damage. Each 15-ounce can covers 10 to 15 square feet and dries to the touch in 60 minutes. The product has a wide spray tip with any-angle technology, so you can easily paint your vehicle while the can is upside down.


This undercoating lays down nicely and produces a smooth low-luster finish if you do several light coats. It covers most surface rust without any issues and dries pretty evenly, even if you overspray in some spots. It's also really easy to use and some really like the reptilian-textured surface it creates.


One downside is this product doesn't appear to be much different from the non-professional version. Also, the coverage isn't particularly thick, and it may splatter a bit when you spray it.

Honorable Mention
POR-15 Rust Preventive Coating Semi-Gloss

POR-15's rust preventative coating permanently stops rust and can be applied on sandblasted, prepped rusted, and seasoned metal surfaces. The formula creates a hard, non-porous finish that doesn't crack, chip, or peel. It protects metal from corrosive contaminants as well as salt and moisture. POR-15 is also resistant to acids and alkalis and repels fuels and oils.


This undercoating covers the vehicle really well and dresses up the metal. Its rust-proofing capability is top notch, and once it sets, nothing can get through it. It’s worth the time and money and provides some peace of mind because you only have to paint the surface once and never worry about rust again.


However, the product is sensitive to UV light and must have a top coat if the surface area is exposed to sunlight. It is also very messy and stains anything it comes into contact with, so you have to be careful when you are using it.

Honorable Mention
Barrier Bond

The Barrier Bond - No Drip Rust-Proofing Coating is a solvent-free rust inhibitor and undercarriage protectant. You can use it on both ferrous and non-ferrous metals. The formula is designed to match OEM equipment and prevent damage caused by corrosion, rust, salt, rock, and gravel.


This is a non-drying formula, so it flows into small, hard-to-reach spaces. It penetrates the base of the metals, provides long-lasting protection, and holds up in various types of weather conditions. The product displaces water and leaves behind a thin translucent coating that won't gum up in the heat. Simply remove dirt, loose rust, and other debris before applying Barrier Bond to the surface, and let it sit for six hours before exposing your vehicle to wet roads.


One downside is that it's a little pricey compared to some other products. Also, it's a non-drying formula, which may not be what you're looking for.

Honorable Mention
Woolwax Undercoating Protection

The Woolwax vehicle undercoating is excellent for automotive use as well as battery terminals, heavy machinery, boat trailers, lawnmowers, garage door tracks, and more. The lanolin-based, nontoxic formula works well in automotive and marine environments to prevent corrosion, provide lubrication, and displace water. It's long-lasting, won't dry out, and penetrates to the base of all metals.


The odor-free formula can be applied using an undercoating gun with 70-90 PSI. It is non-conductive, so it won't harm electrical connections. It's easy to stir and pour from the can, flows well, and holds up over time. Overall, it does what it says it will do and is versatile in its application.


However, the product is a little pricey. Some users also compare it to Fluid Film, but it's not quite as thick.

Honorable Mention
Krylon Automotive Undercoating

The Krylon Automotive Undercoating is one of the least expensive options on our list, so you really can't go wrong if you try this product. Its paintable and flexible formula is designed to protect your vehicle's underside as well as corrosion resistance. It's designed for interior and exterior use and can be applied on metal, wood, and fiberglass.


One can covers 16 square feet, and it dries in 24 hours. You can safely use it on your vehicle's wheel wells, gas tank, frame/supports, weld joints, floor pans, doors, and quarter panels. The matte black finish provides good coverage, adheres well to properly prepared surfaces, and it's easy to use overall.


However, if you get more than one can from different batches, the color may be a little inconsistent. Also, you need to use the entire can at once, or it won't work well.

Honorable Mention
FDC Rust Converter Ultra

The FDC Rust Converter Ultra converts rust into a protective coating and prevents additional corrosion from occurring. The highly efficient formula is for automotive use as well as other applications. Simply remove large rust with a wire brush or sandpaper, clean the area thoroughly, and apply the product with a brush, roller, or sprayer.


If you use this product, you won't have to sandblast, grind, or scrape rust off your car or truck. It's designed to convert rust and corrosion formed by different types of weather conditions. It transforms the rust into a hard, shiny surface, which you can leave as is or paint over with oil-based paint. The company also includes a 30-day money-back guarantee if you're not satisfied with the results.


One gallon covers 500 square feet, but porous or textured surfaces may require an additional application, and it takes 48 hours to cure. It also may take multiple coats to get good coverage.

Tips

  • Use a pressurized washer to clean dust, dirt, mud, and road debris from the underbody before you apply the undercoat. You will have an easier time applying the undercoat when the surface is clean.
  • Before undercoating, do a test spray to see the fan pattern of the bottle. If the coating comes out in a narrow stream, spray the coating up and down to get a proper coat. If it’s a tall fan spray, we suggest that you spray it from side to side.
  • Use a degreaser to remove grease from metal surfaces before applying the undercoat. Most coats do not stick on greased surfaces, no matter how long you leave them to dry.
  • Wear a mask and safety goggles when washing and undercoating your vehicle. That way, you protect yourself from inhaling or coming into contact with the cleaning chemicals and undercoating agent, which could potentially have serious health effects.

FAQs

Q: Which rust proofing is best?

There are several ways to protect your vehicle from rust, including using a drip or dripless oil spray, an undercoating, or an electronic rust protection system. We include many good options in this buying guide.

Q: Does undercoating really work?

Yes. It provides a protective barrier that keeps out moisture, salt, and other substances that can potentially harm the underside of your vehicle.

Q: Is Fluid Film the best undercoating?

Fluid Film is a very effective undercoating used in the automotive and aerospace industries, and its reputation precedes it. However, several other products also do an excellent job of protecting your vehicle’s undercarriage.

Q: Do I have to remove the wheels when undercoating the underbody?

Yes, you do. For starters, you will get a chance to clean the wheels as you clean the underbody. Additionally, you will have enough room to spray the undercoat under the wheel wells and have better access to the underbody.

Q: What should I use to clean my vehicle before applying an undercoat?

You need a pressure washer or an air hose to get rid of dirt from the crooks of the underbody. In addition, you need a rim cleaner, engine cleaner, degreaser, and cleaning chemicals to get rid of rust and other tough stains on metal surfaces.

Q: How often should I give my vehicle a new undercoat?

A fresh undercoat should be applied once a year, and more so before the beginning of the winter season, as your tires tend to flick up moisture, ice, rocks, and dirt at your vehicle's body. If you live near the sea, particles of salt tend to take off some undercoat. In that case, apply a new coat at least twice a year.

Final Thoughts

The Dynatron 544 Dyna-Pro undercoat is our top pick as it provides all-weather protection, spreads easily, and dries quickly.

If you are looking for a budget-friendly undercoat that will keep your undercarriage from being eaten away by rust, try the Penray 4424 rubberized Undercoat.