Rustproofing undercoating

I recently bought a used 2010 accord. It was a rental in NJ, and is corroded underneath… 26000 miles. Mechanic says I need to have him sand down and cover the bottom with rust proofing undercoating so it doesn’t get worse. I live in the south and have little contact with salty roads. Do I need to do this? Thank you

Once rust starts, spraying undercoating on top of it (even if he “sands it down”) usually just makes it worse…The time to apply undercoating was when the car was brand new, before any rust started…

If it is just topical rust, it can be removed. It should then get a rust resistant primer followed by a rustproof coating. But he has to get all the rust off.

Car makers brag how they “rustproof” their cars at the factory, using galvanized panels and electrolyte-coating the lower body in a dip-tank…But sometimes you wonder…maybe your car got built on a day when the tank was empty…

A 2010?
I think he’s hoping to “take you for a ride”. Since you live in the south, you may not be used to seeing what a NJ resident would consider normal.

HOWEVER, … can you say Hurricane Sandy? Countless NJ cars were flooded by Hurricane Sandy in 2012. Many were shipped to other areas of the country to get rid of them. That includes rentals. If yours was a flooded car, nothing will stop the cancer. If it was not, visible rust on the undercarriage is normal after a few years in NJ. Either way, I think the guy telling you this is BS’ing you.

It depends how bad it is.

Does the service being offered come with any kind of warranty? If so, I’d consider it.

I live near the Jersey shore (5 miles inland) and none of my cars have any under body rust. My wife’s 06 Sienna with 7 Jersey winters under it’s belt has no corrosion except a little surface rust on the exhaust system. I would get a second opinion from another mechanic and have the car checked for any evidence of water damage/incursion.

After Sandy, two of my coworker’s cars were totaled from the flooding. The insurance company didn’t attempt to have them repaired. Thousands of cars in the state were totaled and who knows what happened to them.


“The time to apply undercoating was when the car was brand new, before any rust started.”


“I think the guy telling you this is BS’ing you.”

For every bit of rust that is visible, there is much more that is not visible, and which cannot be removed by “sanding”.

My suggestion is…
drive this car until it is no longer safe to operate…
RUN away from this scam-artist mechanic as fast as you can…
have your next car inspected before purchase by a competent mechanic (obviously not the current mechanic!)

Try this: pull the carpet out of the trunk so that you can see all the interior metal. Do you see any dirt or debris that shouldn’t be there? Look closely at the recessed corners. They are the hardest to clean. If so, it might be a flood car. You can also do this with the passenger cabin carpets. If it turns out it is a flood car, you may have some recourse if you bought it from a reputable dealer. Read this article for some very helpful advice.

Something doesn’t seem right. A 2010 with severe undercarriage rust? Econobox cars built within the past 20 years have excellent rust proofing installed by the robots at the factory and seldom need anything more except if drilling or welding has been done on the undercarriage in the meantime. In any event, spraying something to cover the rust is unlikely to work, and is more likely to make it worse. You need to get another independent opinion.

There are plenty of areas that can appear to have a rust coating and be fine. Poking and proding by an experienced body man is the best way to determine if rust areas are structural. Do not cover these areas. If the metal is solid, Use an oil base coating to arrest the rust or paint the areas with grease. Repeat as needed every every two years. All areas treated this way will last indefinitely. Areas covered up with coating, could very easily continue to rust.

It’s entirely possible for a 5 year old car to suffer severe rust issues if there has been flooding involved or the car spent some time in the Rust Belt.

The fact that a mechanic (?) would recommend sanding and spraying some undercoat does come across as a bit strange.

If it can be easily sanded off as the post makes it appear then any rust is not likely to be an issue to worry over.

I would think that a used car…especially one that came from a snow state where salt is used on the roads…would have some of that salt imbedded in areas where it might not get rinsed away in even a heavy rain.
So if you were to do any kind of undercoating prevention, I would think that you would be trapping some of that salt by sealing it in. Yes you are undercoating to seal up some areas, but would be also sealing in some of that salt.
I wouldn’t even consider doing it to a new car, because I could miss a drainage area and water then could not drain like it should.
If it were possible for the manufacture to do it at the factory, then I may be willing to try it because they would know all the areas not to seal up, and it would be done long before anything got trapped in the undercarriage.

I also think that it would be darn near impossible to sand the bottom of a car. There are just too many places that you would miss. It will look pretty from the bottom, but the upper side of any part would be hard to get at.

I’d pass on this service


A much bigger concern is the possibility that this is a Hurricane Sandy car. It’s estimated that over 100,000 cars (maybe WELL over) were immersed to various levels in water, much of the water contaminated with ocean salts and other corrosives, and then moved to other parts of the country for resale. I recommend a Google search on the subject.

Anyway, there’s no way I’d agree to what the shop is offering. It makes no sense at all.

Oddly Im in the other camp. If it was my car I would go ahead and do something about it. At the very least Id hose the undercarriage down really well, preferably with a power washer at one of those do it yourself car washes, really focusing on the wheel well areas and bumpers, among the rest of the underside. Then Id drive it home on the highway and get it dried out well. Then Id go around with a wire brush, like the kind you use to clean a grill and scrub down as much rust as you can, then Id hit it with a rust inhibitor, be it some kind of under coater or atleast some oil like WD40. Be sure to avoid rubber suspension components when you spray anything. Surely leaving it to rust more is worse than giving it attention. Sure thatd be nice to have a time machine and go back in time to when the car was “born” and undercoat everything, but thats not the OP’s current situation.