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ARMORTHANE PROTECTING YOUR WORLD

PROTECTING YOUR WORLD

ARMORTHANE PROTECTING YOUR WORLD

Oil Undercoating:
Products, Amounts
& Real Costs

How oil and lanolin undercoatings work, how Fluid Film, Woolwax, NH Oil and Krown compare, how much product your vehicle needs, and whether to spray it yourself or pay a shop.
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  • Vehicle or equipment type
  • Plow, farm, gravel or highway use
  • Current rust and oil residue
  • Frame, wheel wells or full underbody
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Step 1 of 3
250 sq ft
Coverage of one gallon of a leading lanolin undercoating at 3 to 4 mils
Fluid Film
1 year
Reapplication interval oil undercoating makers recommend for full protection
NH Oil Undercoating
$250–$350
Shop price per application, compact car to 1-ton dump truck, at one New England installer
NHOU of Maine
1M gal
Fresh water that the used oil from one oil change can contaminate, a reason never to spray waste oil
U.S. EPA

Oil undercoating is the closest thing salt-belt mechanics agree on. It is cheap, it creeps into seams that no hard coating can reach, it slows rust that has already started, and it is easy to renew every fall. It also drips, smells for a while, collects dirt and washes off, which is why it has to be redone every year or two. We make hard coatings, including polyurea undercoats, and we still recommend oil undercoating for a lot of vehicles. This guide compares the main oil and lanolin products, shows how much product a vehicle needs, works out DIY versus shop cost, and explains when a hard coating is the better choice.

Quick answer

Oil undercoating is a thin, non-drying oil or lanolin film sprayed on the underbody and inside cavities such as rocker panels, doors and frame rails, where it creeps into seams and displaces moisture. Petroleum-based products are typically reapplied every year; thicker lanolin products such as wool-wax last about two years on exposed areas. At about 250 square feet per gallon, most vehicles need one to a few gallons; the calculator below sizes yours. Shop applications run about $160 to $350; DIY costs well under that after the first year. Never use used motor oil.

What's in this guide
  1. 1What oil undercoating is and how it works
  2. 2Oil undercoating products compared
  3. 3How much oil undercoating you need
  4. 4DIY vs. shop cost calculator
  5. 5How to apply oil undercoating
  6. 6Oil undercoating vs. hard coatings
  7. 7FAQ
The basics

What oil undercoating is and how it works

Oil undercoating is not a coating in the paint sense. It never cures. It is a rust-inhibiting oil, grease or lanolin that stays wet or tacky, so it can migrate into tight seams, spot welds and folded flanges and keep moving after it is applied. One major supplier says a quality product should creep 4 to 6 inches into seams and folds. Because it stays soft, it self-heals when gravel scuffs it, and it does not crack and trap water the way an aging hard coating can.

Petroleum-based

Thin, penetrating, annual

Mineral-oil blends with rust inhibitors, sprayed thin at high pressure so they atomize into cavities. They penetrate best and wash off fastest, which is why makers like NH Oil Undercoating recommend an annual application.

Lanolin-based

Thicker, tackier, longer-lasting

Wool grease products such as Fluid Film and Woolwax. Fluid Film describes its liquid as solvent-free and lanolin-based; Woolwax says it carries more raw lanolin than competitors, making it thicker and more resistant to wash-off.

Both types work by keeping water and dissolved salt off the steel. Neither adds any strength, and neither will stop a stone from chipping paint on the body. They are about the underside and the inside of the vehicle. If chips on rockers and lower doors are your problem, see our rock chip protection guide.

Do not spray used motor oilSome old-timers still "undercoat" with drain oil. Don't. The EPA notes used motor oil can contain toxic chemicals and heavy metals, and that the oil from a single oil change can contaminate one million gallons of fresh water. It also drips off onto your driveway and the road within weeks. Proper undercoating oils are formulated to cling and inhibit rust; used crankcase oil is neither.
Comparison

Oil undercoating products compared

These are the names you will hear at any salt-belt shop counter. We do not sell any of them; the figures come from the makers' and installers' own published data. Click a column heading to sort.

ProductBaseReapply about everyShop price per applicationDIY materialNotes
Fluid Film (Liquid A)Lanolin, solvent-free1 year (exposed areas)Varies by installerAbout $40 per gallon250 sq ft per gallon at 3–4 mils; flash point 315°F
WoolwaxLanolin, no solventsAbout 2 years exposed; years in cavities$249–$399 (one installer's packages)Sold by the gallon and pailThicker, more wash-off resistant; may need heating in cold weather
NH Oil UndercoatingOil-based1 year$250–$350 (car to 1-ton dump)$173–$404 kits (1–5 gal with gun)Sprayed at high pressure to reach seams
Krown (shop network)Oil-based1 yearAbout $160–$180 in-shop (U.S.)Aerosols for touch-upRecommended by Canada's APA; applied through drilled, plugged holes
Used motor oilWaste crankcase oilWeeksNot offered by reputable shopsFree, and a liabilityContaminants and heavy metals; pollutes water
Sources: Fluid Film coverage from its spraying guide, flash point from its Liquid A data sheet, gallon price from The Rust Store; Woolwax characteristics from Woolwax USA, interval and packages from ECC Coating; NHOU interval and DIY kits from NH Oil Undercoating, shop prices from NHOU of Maine; Krown prices from Krown St. Louis, APA view from The Car Guide; used oil from U.S. EPA. Fluid Film's own site does not state an interval; one year on exposed areas is common practice in salt states.
Calculator

How much oil undercoating do you need?

The underbody of a vehicle has far more surface than its footprint: frame rails have four sides, and suspension arms, crossmembers, the fuel tank and the inside of the wheel wells all add area. The calculator multiplies length by width, applies a complexity factor for the vehicle type, adds cavities if you treat them, and divides by published coverage. The complexity factors are our own rule of thumb from estimating jobs, not manufacturer data, so treat the result as a buying estimate.

Product estimate

How many gallons will your vehicle take?

Area = length x width x complexity factor (+25% if cavities are fogged) + overspray. Gallons = area / coverage: about 309 sq ft per gallon at 2–3 mils or 250 at 3–4 mils (Fluid Film figures).

Include
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Area to treat
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Gallons to buy
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1-liter gun refills

Calculator

DIY vs. shop oil undercoating cost

Doing it yourself is cheap on material, but you need a lift or ramps, an undercoating gun, a compressor that keeps up, and a willingness to get oily. NH Oil Undercoating says self-application runs under $100 for cars and under $150 for trucks and SUVs. The calculator compares the full cost over the years you plan to keep the vehicle.

Cost over time

Should you spray it yourself or pay a shop?

DIY = gun (one time) + gallons x price per gallon x applications. Shop = price per application x applications. Defaults: $129 undercoating gun, about $40 per gallon, and a $300 shop job (mid-range of a published $250–$350 list).

Equipment you already own
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DIY total
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Shop total
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DIY saves

Process

How to apply oil undercoating

  1. Pick a dry, warm day before salt season

    Oil goes on and creeps best over dry metal. Late summer or early fall gives it time to work into seams before the first brine. Thick lanolin products may need to be warmed in cold weather to spray properly.

  2. Wash the underbody and let it dry

    Rinse off mud and salt, paying attention to the inside of frame rails and wheel wells. Let it dry overnight if you can. Knock off any loose scale with a wire brush or scraper.

  3. Mask what must stay clean

    Cover brake rotors, pads and calipers, exhaust components, drive belts and the alternator. Fluid Film lists a flash point of 315°F, and exhaust parts run hotter than that, so overspray on a pipe will smoke and smell. Keep oil off brake parts entirely.

  4. Spray with the right pressure

    Air-powered undercoating guns typically run at 70 to 90 psi; one popular gun is rated 60 to 90 psi at 5 to 8 cfm. Airless rigs need around 2,000 psi with a 515 to 517 tip, per Fluid Film. Use a 360 degree wand inside rockers, doors, tailgate and frame rails through existing holes.

  5. Cover the exposed underbody

    Work front to back: frame, crossmembers, floor seams, fuel tank straps, suspension arms, brake and fuel lines, and the backside of bumpers. A thin, even film beats puddles that will just drip off.

  6. Expect some drips, then inspect

    Park it outside or over cardboard for the first days. Check coverage after the first salt event and touch up bare spots. Avoid high-pressure undercarriage washes, which strip the film faster.

Honest trade-offs

Oil undercoating vs. hard coatings

Oil wins

Inside cavities and on older rust

Nothing else gets into closed rocker panels and frame rails. Oil also slows existing surface rust and scale, where a hard coating would need blasting first.

Oil wins

Low cost per year, easy to renew

A gallon or two and an afternoon. If gravel scrubs it off, the next application fixes it.

Hard coat wins

Abrasion and gravel

On plow trucks, farm trucks and anything driven on gravel daily, oil gets sandblasted off. A polyurea undercoat on blasted, primed steel takes the abuse for years.

Watch out

Oil blocks later coatings

Nothing bonds over oil or lanolin. Fluid Film calls for a hot-water or steam detergent wash of at least 120°F before painting. Decide on hard coatings first, then oil the rest.

Many of our applicators combine both: ArmorLiner polyurea on the exposed frame and wheel wells of a work truck, cavity oil inside rockers and rails. Our truck undercoating guide explains that system, the rubberized undercoating page compares it with rubberized products, and our rust proofing guide lays out a full yearly program. For cars and crossovers, start with the car undercoating guide.

Done with the annual mess?

Put a permanent barrier on the exposed steel.

For trucks, plows and equipment that eat through oil every winter, certified ArmorThane applicators blast, prime and spray polyurea on frames, wheel wells and floors, leaving cavities for oil. Tell us what you drive and we will match you with a shop.

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FAQ

Oil undercoating questions

Is oil undercoating better than rubberized undercoating?

For most vehicles in salt country, yes. Oil stays soft, creeps into seams and inside cavities, and does not crack and trap salt water against the metal. Rubberized coatings last longer between applications but must go on clean metal and can hide rust underneath once they crack.

How often do you need to reapply oil undercoating?

Oil-based products are typically reapplied every year; NH Oil Undercoating recommends an annual application. Thicker lanolin products such as Woolwax are often re-treated every other year on exposed areas, and treatments inside enclosed cavities can last much longer.

How much does oil undercoating cost?

Published shop prices run about $160 to $180 per visit at one chain's in-shop rates and $250 to $350 at an independent installer, depending on vehicle size. DIY material costs about $40 per gallon for a leading lanolin product, plus about $129 for an undercoating gun if you do not own one.

Does oil undercoating drip?

Thin oil products drip for a while after application, which is the trade-off for their ability to penetrate. Thicker lanolin and dripless formulas drip less but creep less. Park outside or over cardboard for the first few days after treatment.

Is Fluid Film or Woolwax better?

Both are lanolin-based. Fluid Film is thinner and penetrates easily; Woolwax says it has more raw lanolin and is thicker and more resistant to wash-off, so it lasts longer on exposed surfaces but may need warming to spray in cold weather. Many people use a thinner product in cavities and a thicker one underneath.

Can I use used motor oil as undercoating?

No. The EPA notes used motor oil can contain toxic chemicals and heavy metals, and that the oil from one oil change can contaminate one million gallons of fresh water. It also runs off quickly and has no added rust inhibitors.

Will oil undercoating hurt rubber bushings or brakes?

Keep it off brake rotors, pads and calipers entirely, and mask exhaust parts. Most undercoating oils are formulated to be compatible with typical rubber parts, but check the product data sheet and avoid soaking bushings and boots.

Can you put oil undercoating over rust?

Yes. That is one of its strengths. Remove loose scale first, then apply; the oil penetrates remaining rust and slows it. It will not restore strength to rusted-through metal, which needs to be repaired.

AT
Written by the ArmorThane technical team. We have manufactured polyurea, polyurethane and polyaspartic coatings in Springfield, Missouri since 1989. We do not make or sell oil undercoatings; product figures come from their makers and installers. Updated October 2026.

Sources

  1. Fluid Film, spraying Fluid Film (coverage, pressure, tips, removal): fluid-film.com/spraying-fluid-film
  2. Fluid Film Liquid A technical data sheet: fluidfilm.no/.../Teknisk_datablad_usa_LqA.pdf
  3. The Rust Store, Fluid Film 1 gallon can: theruststore.com/products/fluid-film-1-gallon-can
  4. The Rust Store, Fluid Film undercoating gun: theruststore.com/products/fluid-film-undercoating-gun
  5. Woolwax USA, lanolin undercoating 1 gallon: woolwaxusa.com/Woolwax-lanolin-Undercoating-1-Gallon
  6. ECC Coating, Woolwax details: ecccoating.com/wwdetails
  7. NH Oil Undercoating, DIY kits: nhoilundercoating.com/product/nh-oil-undercoating-diy-kits
  8. NH Oil Undercoating, vehicle undercoating pros and cons: nhoilundercoating.com/vehicle-undercoating-pros-and-cons
  9. NHOU of Maine, pricing: nhouofmaine.com/pricing
  10. Krown St. Louis, price list: krownstl.com/pricing.html
  11. The Car Guide, choosing the best rustproofing treatment: guideautoweb.com/en/articles/79956
  12. U.S. EPA, managing, reusing and recycling used oil: epa.gov/recycle/managing-reusing-and-recycling-used-oil

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