Your car’s firewall does more than just separate you from engine heat and noise—it’s a critical structural component constantly under siege. Every time you drive, this unassuming metal barrier faces a brutal assault: extreme temperature swings from sub-zero mornings to 200+ degree engine heat, corrosive road salt spray, moisture intrusion, and relentless vibrations. While most enthusiasts focus on waxing paint or polishing chrome, the firewall often remains neglected until rust blooms appear or cabin noise becomes unbearable.
Investing in quality firewall undercoating isn’t just about preservation; it’s about transforming your driving experience. The right protective barrier can dramatically reduce heat transfer into your cabin, quiet engine and road noise, and prevent the hidden corrosion that compromises structural integrity and resale value. Whether you’re restoring a classic muscle car or protecting your daily driver, understanding the nuanced world of firewall undercoating materials and application techniques will help you make an informed decision that pays dividends for years.
Top 10 Firewall Undercoating
 | Undercoating In A Can - Black Wax Coating (12) 12oz. Case | Check Price |
Detailed Product Reviews
1. Undercoating In A Can - Black Wax Coating (12) 12oz. Case
Overview: This professional-grade undercoating delivers military-spec rust protection in a convenient aerosol format. The MIL-PRF-62218B approved formula provides a black wax coating that actively prevents corrosion while halting the spread of existing rust. Designed for automotive underbodies, wheel wells, and hidden cavities, this product combines rust inhibition with sound-deadening properties. Each 12-ounce can covers approximately 10-12 square feet, making the 12-can case suitable for multiple vehicles or comprehensive single-vehicle protection.
What Makes It Stand Out: The military certification sets this apart from consumer-grade alternatives, ensuring rigorous performance standards. Unlike typical asphalt-based or rubberized coatings, this wax formula remains flexible and won’t crack or peel over time. Its unique ability to penetrate and encapsulate existing rust prevents further oxidation rather than merely covering it. The sound-dampening characteristics reduce road noise significantly, adding comfort to protection. Most impressively, the manufacturer claims it lasts four times longer than conventional undercoatings, reducing reapplication frequency substantially.
Value for Money: At $74.18 for twelve cans, you’re paying roughly $6.18 per unit—competitive for professional-grade protection. Considering it outlasts standard coatings by years, the long-term value becomes clear. Professional undercoaling services cost $150-$300 per vehicle, making this DIY solution economical for multiple applications. The case quantity suits serious DIYers or small shops, eliminating frequent reordering.
Strengths and Weaknesses: Strengths: Military-grade durability, superior flexibility, stops active rust, excellent sound deadening, long-lasting protection, convenient aerosol application. Weaknesses: Higher upfront cost than budget alternatives, requires thorough surface prep for best results, strong solvent odor during application, wax consistency may attract road grime, coverage per can is moderate.
Bottom Line: This undercoating is ideal for vehicle owners in rust-prone regions seeking professional results without professional prices. While the initial investment exceeds basic rubberized sprays, the extended lifespan and military certification justify the cost. For those willing to invest time in proper preparation, this product offers superior long-term protection that cheap alternatives simply cannot match. Highly recommended for DIY enthusiasts who prioritize durability over convenience.
Understanding Your Firewall’s Vulnerability
The automotive firewall represents one of your vehicle’s most environmentally stressed panels. Unlike underbody components that face direct road debris, the firewall battles a unique combination of engine bay heat, moisture condensation, and chemical exposure. During winter months, snow and slush melt against the hot firewall surface, creating repeated wet-dry cycles that accelerate oxidation. Summer heat causes the metal to expand and contract, stressing factory seam sealers and creating microscopic entry points for moisture. Additionally, brake fluid leaks, engine oil seepage, and atmospheric pollutants create a corrosive cocktail that attacks bare metal. Modern vehicles with turbochargers or performance engines subject firewalls to even higher thermal loads, making protection not just beneficial but essential for longevity.
What Is Firewall Undercoating?
Firewall undercoating is a specialized protective coating applied to the engine-side surface of the firewall panel. Unlike general underbody coatings designed for abrasion resistance, firewall-specific formulations prioritize heat reflection, sound absorption, and corrosion inhibition while maintaining flexibility at extreme temperatures. These products create a durable membrane that seals metal from environmental contaminants, dampens structural vibrations, and often incorporates thermal barriers to reduce radiant heat transfer. Professional-grade applications typically reach 40-60 mils thickness, providing robust protection without interfering with wiring harnesses, HVAC components, or mechanical linkages. The best formulations remain pliable after curing to accommodate metal flex and prevent cracking that would compromise the protective seal.
Key Benefits of Firewall Undercoating
Beyond simple rust prevention, comprehensive firewall undercoating delivers multifaceted advantages. Thermal management ranks among the most immediate benefits—quality coatings can reduce cabin heat by 15-30 degrees Fahrenheit in summer traffic, decreasing AC workload and improving fuel efficiency. Acoustic damping properties significantly muffle engine harmonics, injector ticking, and road noise, creating a quieter, more refined cabin environment. From a preservation standpoint, sealed firewalls resist corrosion that can weaken structural spot welds and create costly repair needs. For restoration projects, undercoating provides a factory-correct appearance while protecting your investment. Perhaps most importantly, maintaining a pristine firewall demonstrates meticulous maintenance to potential buyers, directly impacting resale value and buyer confidence.
Types of Firewall Undercoating Materials
rubberized undercoating
Rubberized formulations remain the most popular choice for DIY enthusiasts and professionals alike. These products blend asphalt or synthetic rubber with solvents and adhesion promoters, creating a flexible, waterproof membrane after curing. The rubberized finish effectively deadens sound and resists chipping from minor impacts. However, not all rubberized coatings handle extreme heat equally—some formulations can soften or emit odors when exposed to turbocharger or exhaust manifold radiant heat. Look for high-temp variants specifically rated for firewall applications, which typically withstand continuous temperatures up to 300°F. Application requires careful surface prep, as rubberized products won’t adhere well to oily or poorly scuffed surfaces.
Asphalt-Based Undercoating
Traditional asphalt-based products offer exceptional moisture resistance and affordability, making them a staple in restoration shops. These coatings create a hard, durable shell that effectively blocks road splash and chemical exposure. Their dense composition provides excellent sound deadening properties, particularly for low-frequency engine rumble. The downside? Asphalt coatings remain relatively rigid after curing, making them prone to cracking if the firewall flexes significantly. They also add considerable weight—sometimes 5-7 pounds per gallon—and can soften in extreme heat. Modern formulations often include reinforcing fibers to improve flexibility, but these products work best on vehicles that don’t see extreme temperature fluctuations or track use.
Wax-Based Undercoating
Wax-based options, including lanolin-enriched formulas, offer a unique approach to corrosion protection. These coatings never fully harden, instead maintaining a self-healing, waxy film that actively displaces moisture. This property makes them exceptionally effective at creeping into seams and protecting hidden crevices where rust typically begins. Wax-based products excel in northern climates with heavy road salt usage, as they resist wash-off better than some alternatives. The trade-off comes in sound deadening—they’re quieter than bare metal but don’t match the acoustic damping of rubberized or asphalt coatings. They also require periodic reapplication, typically every 2-3 years, as the wax film slowly wears away.
Polyurethane-Based Undercoating
Professional-grade polyurethane formulations represent the premium tier of firewall protection. These two-component coatings chemically cure into a tough, flexible membrane that bonds tenaciously to properly prepared metal. Polyurethane offers superior resistance to chemicals, heat, and abrasion while maintaining acoustic dampening properties. The cured film remains flexible across extreme temperature ranges, preventing crack formation. Many polyurethane systems include ceramic microspheres or other insulating additives for enhanced thermal protection. The primary drawbacks are cost—often 3-4 times more than rubberized options—and application complexity, requiring precise mixing, specialized spray equipment, and proper ventilation due to isocyanate content.
Ceramic-Based Thermal Coatings
For maximum heat management, ceramic-based thermal barrier coatings provide unmatched performance. These water-based formulations incorporate ceramic microspheres that create an insulating air gap, reflecting radiant heat rather than absorbing it. While not traditional “undercoating” in the rust-prevention sense, they excel at keeping cabins cool in high-performance applications. Many enthusiasts apply a ceramic thermal coating first, then topcoat with a traditional undercoating for combined benefits. These products typically handle continuous temperatures exceeding 500°F and remain lightweight, adding minimal heft to the vehicle. Their sound deadening properties are moderate, and they require immaculate surface preparation for proper adhesion.
Critical Features to Evaluate
Corrosion Resistance
The primary function of any firewall undercoating is preventing rust, but not all products deliver equal protection. Evaluate formulations that specifically mention salt spray testing results—quality products often exceed 1000 hours in ASTM B117 testing without failure. Look for active corrosion inhibitors like zinc phosphate or barium metaborate in the ingredient list. These compounds chemically neutralize rust formation rather than simply creating a physical barrier. The coating’s moisture vapor transmission rate also matters; lower rates indicate better sealing properties. For maximum protection in harsh climates, consider products that meet military specification MIL-PRF-16173 for corrosion prevention.
Heat Resistance and Thermal Protection
Firewall temperatures routinely exceed 200°F near exhaust components and can spike much higher during spirited driving. Verify the product’s continuous operating temperature rating, not just peak exposure limits. Premium formulations include heat stabilizers that prevent degradation and maintain protective properties at elevated temperatures. For forced-induction vehicles, prioritize products rated for at least 350°F continuous use. Thermal conductivity values indicate how effectively the coating insulates—lower values mean better heat blocking. Some advanced formulations integrate phase-change materials that absorb and dissipate heat more effectively than traditional barriers.
Sound Deadening Properties
Sound deadening performance depends on both mass and damping coefficient. Heavier coatings generally block more noise, but advanced viscoelastic polymers can achieve superior results with less weight. Look for products specifying Sound Transmission Class (STC) or damping factor ratings. Multi-layer systems combining a vibration-damping base with a mass-loaded topcoat often outperform single-product applications. The coating’s thickness directly impacts acoustic performance—aim for at least 60 mils in critical areas for noticeable noise reduction. Remember that complete coverage matters; even small uncovered areas can act as sound leakage points.
Flexibility and Adhesion
A coating that cracks or delaminates provides zero protection. Test flexibility by applying a sample to a flexible metal strip—quality products should bend 180 degrees without cracking. Adhesion testing should include cross-hatch tape tests after thermal cycling. The best formulations maintain elasticity from -40°F to 300°F, accommodating metal expansion without losing bond. Pay attention to elongation-at-break specifications; values above 200% indicate excellent flexibility. For vehicles with extensive spot-welding or seam work, choose products with high thixotropy that won’t sag on vertical surfaces but still flow into crevices.
Application Method
Your chosen application technique significantly impacts final results. Aerosol cans offer convenience for touch-ups but rarely achieve adequate thickness or uniformity for full firewall coverage. Compressor-driven spray guns provide professional results but require 50-60 PSI and proper tip selection (typically 1.8-2.2mm). Brush or roller application works well for wax-based and some rubberized products, allowing controlled thickness buildup. Two-component polyurethanes demand plural-component spray equipment with precise mixing ratios. Consider your workspace—solvent-based products require excellent ventilation and respiratory protection, while water-based ceramics offer easier cleanup.
Curing Time and Process
Understanding cure dynamics prevents premature use and ensures maximum protection. Solvent-based products skin over in 30-60 minutes but may require 7-14 days for full cure, during which time they remain vulnerable to solvents and heavy moisture. Water-based ceramics cure faster but need low humidity for optimal results. Polyurethane systems have working times of 30-90 minutes after mixing, with full cure in 24-48 hours. Temperature and humidity dramatically affect cure rates—most products require temperatures above 60°F and relative humidity below 70% for proper film formation. Rushing the cure process by forcing heat can trap solvents, leading to bubbling and reduced adhesion.
Application Methods Compared
Spray Application
Spray application delivers the most uniform coverage and professional finish when executed correctly. High-volume low-pressure (HVLP) spray systems minimize overspray while depositing adequate material. Maintain 6-8 inch distance from the surface, applying in 3-4 light passes rather than one heavy coat to prevent sagging and solvent entrapment. For detailed areas around wiring grommets and HVAC penetrations, use masking tape and paper to protect sensitive components. Spraying allows you to build thickness gradually, achieving the 40-60 mil target without runs. The downside includes significant prep work, ventilation requirements, and equipment costs. Overspray can damage nearby surfaces, making garage spraying risky without proper containment.
Brush or Roller Application
Brush and roller methods offer superior control for novices and work exceptionally well with thicker, high-build formulations. Use natural bristle brushes for solvent-based products and synthetic brushes for water-based ceramics. Roller application with 3/8-inch nap rollers speeds up coverage on broad firewall sections while forcing material into contours and seams. This technique excels at building thickness in specific areas needing extra protection, like lower firewall sections near wheel wells. The trade-off is texture—brushed finishes lack the uniform appearance of spray applications and may show stroke marks. However, for concours-correct restorations, some judges prefer the authentic brushed appearance replicating factory applications from the 1960s and 70s.
Preparing Your Firewall for Undercoating
Proper preparation determines 80% of your final result. Begin by removing all loose rust and scale using a needle scaler or wire wheel on a drill—surface rust is acceptable, but flaky oxidation must go. Follow with 80-grit sandpaper to scuff the entire surface, creating a mechanical tooth for adhesion. Degrease thoroughly with wax-and-grease remover, not just soap and water, which can leave residues. Pay obsessive attention to seams and spot welds, using a dental pick to remove old seam sealer and debris. After cleaning, wipe down with a tack cloth and allow complete drying—preferably 24 hours in a heated, dry environment. Mask off all wiring, brake lines, and mechanical components with high-quality automotive masking tape and plastic sheeting. Finally, apply a rust-converting primer to any bare metal spots before undercoating application.
Common Application Mistakes to Avoid
The most frequent error is inadequate surface preparation—skipping degreasing or rust removal guarantees premature failure. Applying too thick a single coat causes solvent entrapment, leading to bubbling and soft spots that never properly cure. Another mistake is coating over existing rust without conversion treatment; the undercoating merely seals moisture in, accelerating corrosion. Temperature matters—applying below 60°F prevents proper flow and adhesion, while applying above 85°F causes solvents to flash off too quickly, creating a porous, weak film. Don’t forget to remove masking tape while the coating is still slightly tacky; waiting until full cure can pull the entire film off the edge. Finally, many DIYers neglect to protect themselves—solvent-based undercoatings contain hazardous VOCs requiring proper respirators, not just dust masks.
Maintenance and Longevity
Even the best undercoating requires periodic inspection and maintenance. Annually check for cracks, chips, or separation, particularly after winter driving seasons. Touch up damaged areas immediately using the same product type—mixing incompatible chemistries leads to delamination. Wash the firewall gently during engine detailing, avoiding high-pressure sprays that can lift coating edges. Wax-based products need reapplication every 2-3 years, while quality polyurethane systems can last 7-10 years with proper initial application. Monitor areas near exhaust components for heat-induced hardening or discoloration, which indicates the coating is nearing its thermal limits. For vehicles stored long-term, consider a light refresher coat every five years as preventative maintenance, even if no visible damage exists.
Cost Considerations and Value
Budget-friendly aerosol rubberized undercoating might cost $30-50 for a single-vehicle application, but often requires reapplication every 2-3 years and provides minimal sound deadening. Mid-range brushable rubberized or asphalt-based products run $80-120 and offer 4-6 year protection with better performance. Professional polyurethane systems cost $150-250 in materials but deliver decade-long protection when properly applied. Ceramic thermal coatings add another $100-150 but provide unmatched heat management. Factor in hidden costs: surface prep supplies ($50-100), spray equipment rental ($40-75/day), and PPE ($30-60). Professional application typically ranges $400-800 depending on vehicle complexity. While premium materials cost more upfront, their extended lifespan and superior performance often make them more economical over the vehicle’s lifetime, especially for high-value or restoration projects.
Professional vs. DIY Application
Professional shops offer expertise, proper equipment, and controlled environments that virtually guarantee excellent results. They can handle two-part polyurethanes safely, achieve uniform thickness, and properly mask complex modern engine bays with multiple sensors and modules. The investment makes sense for concours restorations, high-end vehicles, or when dealing with extensive rust repair. However, DIY application remains viable for enthusiasts willing to invest in proper prep and equipment. The learning curve is manageable, and the satisfaction of personal involvement is significant. DIY works best with single-component rubberized or wax-based products applied via brush or roller. Consider your workspace—a clean, heated garage with exhaust ventilation is non-negotiable for spray applications. Also assess your comfort with chemical safety; isocyanate-containing polyurethanes demand strict protocols that many home users cannot safely meet.
Environmental and Safety Factors
Modern undercoating formulations increasingly address environmental concerns, but vigilance remains necessary. Volatile Organic Compounds (VOCs) in solvent-based products contribute to air pollution and pose health risks during application. Many jurisdictions restrict VOC content, driving development of water-based alternatives that perform nearly as well. Always check local regulations before purchasing—some areas prohibit high-VOC products for consumer use. Disposal presents another challenge: never pour leftover undercoating down drains or into regular trash. Hardened waste can go to landfill, but liquid material requires hazardous waste collection. From a safety perspective, isocyanates in two-part polyurethanes can cause severe respiratory sensitization; use only in well-ventilated areas with supplied-air respirators. Even single-component products require organic vapor cartridges and eye protection. Keep fire extinguishers nearby—solvent vapors are flammable, and overspray can drift to ignition sources.
Compatibility with Modern Vehicle Systems
Today’s engine bays house sensitive electronics that didn’t exist when traditional undercoating was developed. ABS modules, PCM computers, and myriad sensors can malfunction if coated or contaminated with undercoating overspray. Modern formulations are electrically non-conductive, but that doesn’t mean they’re electronics-friendly. Avoid coating any ground straps, as undercoating can create resistance in electrical pathways. Be extremely cautious around oxygen sensors and wiring connectors—masking tape alone may not prevent solvent wicking into connections. Some manufacturers specifically void warranties if undercoating is detected on certain components. For hybrid or electric vehicles, firewall undercoating requires extra consideration due to high-voltage systems and battery thermal management. Always consult your vehicle’s service manual and consider professional application for vehicles with complex electronic architectures.
How to Choose the Right Undercoating for Your Needs
Start by honestly assessing your vehicle’s use case and environment. Daily drivers in salt-belt states need maximum corrosion protection—prioritize wax-based or high-quality rubberized products with active rust inhibitors. Performance cars and turbocharged vehicles demand thermal management first; consider ceramic thermal barriers topped with polyurethane for comprehensive protection. Restoration projects seeking factory-correct appearance should research original equipment specifications—many classics used asphalt-based coatings applied with specific textures. Budget-conscious owners can achieve excellent results with mid-tier rubberized products applied meticulously. Climate matters—extreme cold requires flexible formulations that won’t crack, while desert heat demands high-temp ratings. Finally, evaluate your own capabilities; if you lack proper ventilation or spray equipment, brushable products offer a safer, more controlled application method that still delivers professional-grade protection.
Frequently Asked Questions
How long does firewall undercoating typically last before needing reapplication?
Quality single-component rubberized undercoating generally lasts 4-6 years under normal driving conditions, while premium polyurethane systems can protect for 10+ years with proper initial application. Wax-based formulations require reapplication every 2-3 years as they slowly wear away. Longevity depends heavily on climate, driving habits, and initial prep quality. Vehicles in salt-heavy winter regions or those frequently exposed to extreme heat may need touch-ups sooner. Annual inspection is crucial—catching and repairing small cracks or chips immediately prevents widespread failure and extends overall service life.
Can I apply new undercoating over an existing factory coating?
You can, but success depends on the original coating’s condition and compatibility. First, thoroughly clean and scuff the existing coating with 180-grit sandpaper to create adhesion. Test a small area for compatibility—apply a quarter-sized patch, let it cure for 48 hours, then attempt to peel it off. If it lifts easily, the chemistries aren’t compatible and you’ll need to remove the old coating. Never apply over flaking, bubbling, or heavily oxidized existing coating; strip it completely first. For factory undercoating in good condition, a light scuff and clean is sufficient for most modern rubberized or polyurethane products to adhere properly.
Will firewall undercoating really make my cabin noticeably quieter?
Yes, but expectations must be realistic. A properly applied 60-mil coating can reduce engine and road noise by 3-6 decibels, which the human ear perceives as a significant improvement. The biggest gains come from reducing low-frequency rumble and high-frequency injector ticking. However, undercoating alone won’t transform a noisy vehicle into a luxury sedan—it works best as part of a comprehensive sound management strategy including door panel damping, floor pan treatment, and sealing air leaks. The firewall is a major noise path, but not the only one. For maximum quiet, combine undercoating with a dedicated mass-loaded vinyl barrier on the cabin side of the firewall.
Is it necessary to remove the engine to properly undercoat the firewall?
Not for most vehicles, though it certainly makes the job easier. With proper masking and careful application, you can achieve excellent coverage with the engine in place using long-reach spray wands and flexible extension nozzles. Remove what you can easily—battery, air intake, and some wiring harnesses—to improve access. For concours-level restorations or when dealing with severe rust requiring welding repair, engine removal is justified. Modern engine bays are packed tightly, making complete coverage challenging but not impossible. Professional shops often use borescope cameras to verify coverage in hidden areas. If you’re DIYing, take your time, use multiple thin coats, and accept that 95% coverage is vastly better than 0%.
How do I handle wiring harnesses and sensors when undercoating?
Meticulous masking is non-negotiable. Use high-quality automotive masking tape and plastic sheeting to completely cover all wiring, connectors, sensors, and modules. For wiring that runs close to the firewall surface, consider wrapping it in aluminum foil first—this prevents solvent wicking along the wire insulation. Never spray directly at connectors or grommets; solvent can seep inside and cause intermittent electrical gremlins that are maddening to diagnose. After coating, remove masking while the material is still slightly tacky to prevent creating hard edges that could peel. If you accidentally get undercoating on a connector, let it dry completely, then carefully scrape it off with a plastic tool—never use harsh solvents that could damage connector housings.
Can firewall undercoating cause any long-term problems?
When properly applied to a clean, dry surface, quality undercoating causes no issues. Problems arise from application mistakes: coating over rust traps moisture and accelerates corrosion; covering factory drain holes prevents water escape, leading to hidden rust; coating ground straps creates electrical resistance. Another concern is heat-induced odor—some low-quality rubberized products emit a rubber smell when the firewall gets hot, which can permeate the cabin through HVAC intake. Using coatings not rated for continuous high heat can lead to softening, sagging, or even smoking near exhaust components. Always choose products specifically formulated for firewall use and follow manufacturer temperature ratings to avoid these issues.
What’s the best time of year to apply firewall undercoating?
Early fall provides ideal conditions—temperatures between 65-75°F with low humidity allow proper curing before winter salt exposure begins. Spring works well too, after winter’s moisture has dried but before summer heat arrives. Avoid application when humidity exceeds 70% or temperatures drop below 60°F, as cure will be compromised. Never apply in direct sunlight or on surfaces hotter than 90°F. The coating needs 24-48 hours of dry conditions to develop initial cure strength, so check the weather forecast and garage the vehicle during this critical period. If you’re in a year-round warm climate, any time works as long as you control temperature and humidity in your workspace.
How does firewall undercoating differ from underbody undercoating?
Firewall undercoating prioritizes heat resistance and sound deadening over abrasion resistance, while underbody coatings must withstand constant gravel impact and road debris. Firewall formulations typically have higher solids content for acoustic damping and include thermal stabilizers for engine bay temperatures. Underbody products emphasize flexibility and chip resistance for suspension movement and stone impacts. You can use firewall coating on underbody areas, but it’s not optimized for that abuse and will wear faster. Conversely, using underbody coating on the firewall often results in inadequate heat resistance and potential odor issues. For best results, use purpose-formulated products for each application, though some premium polyurethane systems perform excellently in both locations.
Will undercoating affect my vehicle’s resale value?
Professional-quality undercoating almost always enhances resale value by demonstrating meticulous maintenance and preventing the hidden rust that scares knowledgeable buyers. However, sloppy DIY application with overspray on components or obvious runs and sags can raise red flags about overall care quality. For collector cars, era-correct undercoating type and texture is crucial—incorrect modern coatings can actually detract from value in concours judging. When selling, mention the undercoating in listings and maintenance records, noting the product type and application date. Provide photos of the clean, protected firewall. Buyers in rust-prone regions particularly value this preventative measure, often justifying a $500-1000 premium over similar vehicles without protection.
Are there any vehicles that shouldn’t have firewall undercoating applied?
Most vehicles benefit from firewall protection, but exceptions exist. Some ultra-high-performance cars with extensive carbon fiber or composite firewalls shouldn’t be coated—these materials have different expansion rates that can cause delamination. Certain exotic vehicles use the firewall as a structural element in crash engineering, and adding coating mass could theoretically affect designed crumple zones (though this is extremely rare). Vehicles with known active firewall leaks should have the leaks repaired before coating, as sealing water inside guarantees rust. Finally, if your car is under factory warranty and the manufacturer specifically prohibits undercoating (check your owner’s manual), violating this could void corrosion coverage. For nearly all mainstream vehicles, especially those driven in harsh climates, firewall undercoating is strongly recommended.