Hi, Top Gear. I just want to ask when and how frequently should we have our cars underwashed. Is regular underwashing sufficient enough to prevent rust in the undercarriage rather than the expensive undercoat and rust-proofing? Thank you!
What I do is have my undercarriage cleaned whenever I encounter conditions that allow mud and grime to cake up under the wheel wells and other areas. Otherwise, the cursory cleaning whenever the rest of the car is washed is all that I do. And the only cars that I've had to undercoat the undercarriage were the ones that I've driven competitively during dirt rally and rally cross.
Most, if not all, new cars come from the factory with a certain amount of anti-corrosion coating already applied in the areas that matter, by the way.
Now, for the undercoat and rust-proofing.
The turning point when undercoating became obsolete as a means of preventing cars from turning to rust powder came in the '90s, when the advances in paint-finishing technology for brand-new cars allowed the newly built bare chassis to be electrocoated before final painting. The e-coat allowed for a much better substrate for the paint to adhere to while giving the benefit of much better protection against corrosion.
In the ideal world of smooth roads free from bomb crater-like potholes, mountain-sized speed bumps, boulder-sized road debris, and endless magically appearing road “rehabilitation” projects, you wouldn't need to have to undercoat your vehicle's under chassis. In fact, if you drive conscientiously and avoid driving conditions that might unnecessarily scrape the bottom of your vehicle, you wouldn't need to at all.
Most car bodies built after the process of e-coating are very durable and corrosion-resistant. It takes a lot more than what a careful driver will encounter to remove that e-coat. Having said that, undercoating your chassis does give you an additional measure of protection as whatever abrasive object you may encounter needs to go through that much more material before it takes away the paint and e-coat.
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