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Engine & Transmission Tips • Issue 31

Vehicle Undercoating


Undercoating your vehicle. Is it worth it? And, can you do it yourself?

uth rust corrosionIf you have a new car, you’re covered with undercarriage protection direct from the factory, so you really don’t need to add undercoating. Some auto manufactures will even void their own factory corrosion-perforation warranty if the vehicle is undercoated by a third party, specifically meaning the dealership.

BUT, if you live in a harsh climate, additional undercoating or rust protection may be a necessity if you want to keep your vehicle more than a few years.

Under coating offers two types of protection. The first is to reduce corrosion, and then second is to dampen outside “road” noise. Each goal requires a different type of product. Just understand, applying undercoating to an older car can be detrimental if the surface isn’t prepared properly. Sealing rust under an undercoating gives it the opportunity to continue to corrode the metal it’s in contact with, so if you don’t want to bother preparing the underside of your vehicle, either get someone else to do it, (professionally), or move on.


Different Types of Undercoating aka Rustproofing:

Oil-Based Undercoating – These are an effective, short-term method of preventing damage caused by things like winter road salt. It is a thin, oil coating that can creep into microscopic spaces and displace moisture, the food for rust. These coatings need to be reapplied regularly.

Rubberized Undercoating – Generally this economical product involves spraying a tar-like substance to the surfaces on the underside of your vehicle. They are superior in adding sound-deadening to the undercarriage. This product dries over time, and can be painted. It also maintains a reasonable amount of flexibility. But the areas to be covered MUST be cleaned, as any trapped rust will continue to corrode and expand under the coating. It is commonly recommended NOT to use rubberized undercoating as it traps moisture and promotes rust from within.

Wax-Based Undercoating – These products are solvent-based and both penetrate and seal. Their protection can span up to 15 years. They require less preparation, are more adept at stopping mild rust from progressing, and they maintain a flexibility over time, so they won’t crack or develop fissures where moisture can penetrate. But again, the surface to be coated must be free of rust because this is another moisture-trapping product.

Asphalt-Based Undercoating – This product has similar properties to the Wax-Based products but is less expensive and offers only about five years of protection. You also have to apply more coats, as the efficacy is increased with thickness of the material. And it does eventually dry to the point of cracking so be prepared to re-coat it regularly. But as an oil-based coating, it will offer moisture-wicking properties when first applied.

uth fluid film applicationFluid-Film Undercoating – Also known as Drip Oil Undercoating, this is a thin, oil-based spray that penetrates virtually every cavity of the vehicle. This is considered the most effect type of undercoating available. Drip or weep holes do need to be cleared, or sometimes drilled, as the product is designed to be sprayed inside door and fender cavities and be allowed to drip out as it coats everything. The upside is it is extremely effective and not overly expensive. The down side is you need to have it reapplied almost annually.

Hard-Coat Undercoating – As the name implies, this product cures to a rock-hard coat offering a smooth finish and excellent chip protection. This product can also be color-coat painted, which wax, oil, and asphalt-based coatings cannot. This is more of a beauty finish for firewalls and wheel wells of antique or vintage restored vehicles where the surface may be viewed by a judge or the public in general.


Steps to prepare your older vehicle for undercoating:

Clean the underside of your vehicle:

  1. Protect Yourself – First, get yourself a pair of extra long chemical-resistant rubber gloves and chemical-resistant goggles. NOTE: We adamantly recommend you wear full-coverage hand/arm and eye protection, (i.e. goggles), when doing work like this, as more often than not, you’re upside down and gravity is going to fling or drip gunk down your arms, or right in your face.
  2. Pressure wash – You’ll get a lot of the superficial junk off the undercarriage with a good pressure washing utilizing a strong degreaser.
  3. Remove any existing undercoating – Do this by applying Undercoating Removal Spray and break out your scraper. Get the thick stuff off first and then try your wire wheels or coarse Scotch Brite abrasive pads.
  4. Sandblast or grind areas of obvious corrosion – You can get a small sandblaster at Harbor Freight and make quick work of cleaning up the underside of your vehicle. You can also get an extensive array of wire wheels that can be attached to a drill. Finally, a grinder can be used, though it’s bulky and heavy to work with.
  5. Prime and paint any exposed bare metal – Use high-zinc primer and flat black automotive spray paint. Allow both ample time to dry/cure.


uth undercarriageApplying Undercoating yourself:

NOTE: We really don’t recommend doing this task yourself, as a professional will do a much better job.

You will need the following:

  1. Enough drop cloths to cover under your vehicle up to six feet out from the sides.
  2. Crappy, disposable clothes with long sleeves and pants.
  3. Low-tack masking tape and craft paper to protect surfaces you don’t want to coat.
  4. Personal protection equipment, see above.
  5. SAFE vehicle lifts, ramps, stands, etc.

Spread the drop cloths out on your driveway, lawn, or wherever and understand that whatever undercoating you’ve chosen to use, will get all over the ground and you. And they are all caustic.

Get your ramps/lifts in place and jack up your car as high as is safely possible.

Tape with paper, all the surfaces you do not want overspray to reach. You may want to remove your wheels altogether and cover the exposed parts.

Prepare yourself with full-coverage clothing, gloves, and goggles. A base-ball cap might also be advantageous.

As you spray your product, start as high up in the undercarriage as possible. Don’t linger with the spray, hoping it will drip down into crevasses, unless it’s specifically a drip oil product. Light repeated coats from different angles will produce better results than trying to lay on one thick coat.

In closing, if you live in a harsh climate with lots of rain and snow, an oil-based undercoating is your best bet. The only caveat is that it must be applied every year. Our customers in Canada know all about going to Krown Undercoating to get their Drip Oil aka Fluid Film undercoating applied every Fall. If you’ve got a vehicle you love, treat the underside as well, or better, than you treat the top side.


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