10 Best Undercoatings 2025 in the United States

The Eastwood Matte Black Rust Encapsulator Plus is designed as a rust-stopping undercoating that works by penetrating rusty metal to prevent further corrosion. It offers strong durability and heat resistance up to 350°F, making it suitable for engine bays and brake areas. Application is flexible—you can brush, roll, or spray it on after cleaning the surface, which may require some prep work like removing loose rust and grease.

It dries fairly quickly within about 2 hours, helping to speed up your project. The finish is a matte black that’s water-resistant, ideal for both indoor and outdoor metal surfaces. The product contains chemicals that require careful handling and protective gear and is not the most environmentally friendly option.

It comes in a large 1-gallon size, which is great for bigger jobs but might be overwhelming if you only need a small amount. For those needing a tough, heat-resistant rust encapsulator and comfortable following safety instructions, this product provides reliable long-term protection.

Fluid Film Rust and Corrosion Protection is a thick, lanolin-based undercoating designed to prevent rust and lubricate metal parts. Its unique formula uses wool-wax and refined oils, making it strong against corrosion while also able to penetrate deep into metal surfaces. This makes it especially good for protecting and loosening stuck bolts or heavily corroded parts, which is a big plus if you work with machinery or vehicles in tough environments like marine or heavy equipment.

Because it’s solvent-free and non-toxic, it has a lower environmental impact compared to many chemical undercoatings, which is helpful if you prefer safer products. On the downside, Fluid Film is a thick liquid that doesn’t dry hard like some undercoatings, so it stays a bit tacky and might attract dirt if used in dusty areas. Application can require a bit more effort to spread evenly, and it doesn’t set quickly, so you’ll need to allow more time before your equipment can be used again.

Its durability is excellent for long-term rust prevention, but if you want a hard protective shell or faster drying time, this may not be the best fit. The large 5-gallon pail suits heavy users or professional settings rather than casual or small-scale use. Fluid Film is a strong choice if you prioritize deep rust protection, lubrication, and a safer environmental profile, but it’s less ideal if you want a dry, fast-setting undercoat.

Fluid Film Rust and Corrosion Inhibitor comes in a convenient 6-pack of 11.75 oz aerosol cans, including helpful accessories like a spray extension wand and an ergonomic Can-Gun tool. This undercoating spray is oil-based with a unique lanolin formula that deeply penetrates metal surfaces to provide long-lasting rust protection without drying out or becoming sticky. It stays slick, which helps it keep lubricating and defending against corrosion, making it a solid choice for vehicles, machinery, or any metal parts exposed to harsh weather.

Application is user-friendly thanks to the included trigger tool that reduces hand strain, and the extension wand helps reach tight spots easily. It dries quickly to a semi-gloss black finish and is waterproof, so it holds up well outdoors. One standout feature is its eco-friendly, non-toxic makeup, which means it’s safer for you and the environment compared to many other undercoatings.

Because it remains oily rather than hardening into a solid film, it might attract dust or dirt over time and could require reapplication to maintain protection. Its oil-based nature may not suit those wanting a dry, paint-like undercoat. For those seeking a versatile, long-lasting rust barrier that's gentle on the environment and easy to apply, Fluid Film is a strong candidate, especially if comfortable with a slick finish rather than a fully hardened coating.