Probably the most divisive home auto product on the market today perhaps sharing the top spot with fuel additives is aerosol-based undercoating.
The application is simple: shake, pop the top, and spray. Voila.
However, multiple automotive forums show that there is a clear divide between advocates of canned undercoatings and its many opponents.
This controversial process has curiously led to increased corrosion and rust on the underside of some vehicles while performing exceptionally on others...but, how is this possible?
In short, not all aerosol-based undercoatings are created equal.
There are many, many brands and varieties of undercoating: Rustoleum, 3M, Eastwood, Undercoating in a Can, etc. etc. The verdict is still out which ones actually work??
The problem is, depending upon the circumstance and the degree to which rust is present on a vehicle's underbody, the results greatly vary, especially after months and months of unpredictable weather.
Most of these brands (Rustoleum, 3M, Eastwood) produce rubberized versions of undercoating in a can. These types of coatings have a dried film that is often quite hard, arid, and brittle.
Make sure you choose an undercoating and not the automotive equivalent of this confectioner's treat. (Blue Peanut Brittle)
While laying it on thick seems like the best practice, a lot of users have experienced some serious issues with this type of undercoating. For example, the following is a quote from a forum user, GodwinAustin, on www.yotatech.com, who'd used rubberized undercoating on his 2004 Tacoma:
In my experience, using rubberized undercoating is just a bad idea if you are in a rust prone area. I applied the stuff to parts of my frame to protect' it from rust. Well I noticed some rust under some of the coating this summer and scraped a bunch of it off and there was rust in areas that were clean before the application so basically water can get under the stuff and when it does, and it will, rust will occur at a real fast rate.
There is nothing more potentially damaging to your vehicle than one simple combination: water & air.
User GodwinAustin highlights the exact problem with rubberized undercoatings. The rubber characteristics of the dried films of these products makes the final product look exceptionally smooth and durable however, this same property (more often than not a heightened degree of cohesion) allows for air and moisture to seep underneath the coating.
So while the coating is very durable, with its molecules bonded tightly to one another, the adhesive abilities of rubberized formulas are increasingly lacking. In fact, the added cohesion can exasperate the problem, allowing rust to spread at alarmingly fast rates when it is designed to do the exact opposite.
Rubberized undercoatings do a great job of keeping existing rust all covered up and looking sharp, but it does absolutely nothing to slow down the spread of rust and corrosion sometimes the trapped air and moisture may even contribute to rust formation as was the case with GodwinAustin.
As far as aerosol-based options, the only other varieties seem to be oil-based coatings and solvent-based wax coatings.
Oil-based coatings are a bit of a misnomer often not a mass produced product, motor oil and other oil types (including vegetable oils) are simply repurposed to act as an undercoating film for the underbody of vehicles. Oils work by keeping out the air and moisture, and have long been a home remedy for rust formation underneath vehicles. While this works well to prevent rust from starting, existing rust may indeed continue to eat away at the frame of your vehicle.
Finally, solvent-based waxes are used as rust prevention primarily in industrial situations. Undercoating in a Can is a solvent-based wax packaged in an aerosol can while it may take an entire case to coat your vehicle, it has so far shown outstanding promise as the market leader.
The paraffin content of the film provides exceptional adhesion to metal surfaces, even those plagued by existing rust (though, if rust is loose, you should always try to remove as much as you can before spraying any undercoating). The film allows for underlying moisture to evaporate during the drying process additionally, the film never truly dries, leaving a very firm, but somewhat tacky finish. While this may seem undesirable for some, this property is exactly what allows for self-healing, which prevents the spread of new rust when an area may become exposed via flying gravel or other debris. Cutting off oxygen and water, solvent-based waxes are exceptionally good at stopping rust from spreading.
As is always the case with undercoating, no topical product can ever remove existing rust. The only way to completely remove rust is via blast media, or sandblasting, which exposes raw, fresh metal underneath the layer of rust. However, nothing corrodes quicker than fresh metal, so it is recommended after blasting to use a coating product.
Which one will you choose?