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The hardest part of using an automatic car wash—the kind where machines do all the work while you wait—might be making the right choice from a staggering menu of options. Should you pay more for spray-on wax? Is an undercarriage wash worth it? And what the heck is “wheel brite”?

Whether you’re washing off a layer of pollen or stubborn dirt, it’s confusing. That’s why CR checked in with our own experts as well as paint specialists, car-wash owners, professional detailers, and soap manufacturers to find out just what goes on behind the scenes at a car wash.

Although extras like wax and tire cleaner may make your car look better, we found out that it won’t hurt your car to skip them—and it might just save you some cash.

How Often Should I Wash My Car?

Cleaning a car isn’t just about cosmetics. According to John Ibbotson, Consumer Reports’ chief mechanic, regular washing can help protect your car’s finish. “You need to remove that road grit and residue left from rain and birds, or it can lead to damaged paint and corrosion,” he says. That’s true even if your car has sat outdoors in a driveway or parking space without being driven.

What does regular washing mean? Dennis Taljan, director of global technology at the paint company PPG has a simple rule of thumb: “When you see stuff on it, wash it,” he says.

Taljan points out that today’s cars aren’t as susceptible to rust, peeling paint, and faded clearcoat as older cars used to be, but they still need thorough cleanings to keep grime, salt, and mud from accumulating in places where they can get stuck.

“It’s really about getting the stuff off the nooks and crannies,” he says. “If you get a stone chip or something that takes the coating away, and you’ve got that salt or grime built up, it can start to attack the metal.”

Car washes also keep that grime and dirt from settling into paint, says Paul Lamberty, who was a technical manager for automotive coatings at BASF before retiring recently. “The dirt will actually sweat into the paint film, and that detracts from the gloss.”

The same goes for bird droppings, which can degrade the finish on paint as they dry, and bugs, which release amino acids that can damage a car’s clearcoat. “It’s a very slow process, but it will etch into the film,” Lamberty says.

How Can I Tell If a Car Wash Is Any Good?

Drivers can check out what online reviewers have to say, but “the best thing is just to pull in and look at it,” says Patrick Mosesso, owner of the Auto Bright Car Wash in Framingham, Mass. Eyeball the cars coming out of the wash: Are they clean? Is the building well-cared for?

If the owners take care of the outside of the wash, chances are they’ll also be investing in good equipment inside, Mosesso says.

Taljan agrees. “I look for a car wash that’s well-maintained,” he says.

Car washes that seem really cheap may have a hidden cost. Gina Budhai, a professional detailer and managing partner at Car Pool Detail in Richmond, Va., cautions that those car washes may save you money but hurt your car.

They might not replace dirty towels, or use more caustic soaps that remove dirt easily but aren’t as good for a car’s finish. “They’re looking for added revenues,” she says. “What kind of brushes are they using? Are they too harsh for the car? Are they using the cheapest soaps?”

They also might be rushing the job. Cheap car washes might get straight to scrubbing, but a good car wash will apply soap at the beginning and give it a few seconds to soak in.

Without presoaking, dirt will be dragged across a car’s surface instead of being washed off, Mosesso says. “That soap is not just soap,” he says. “There’s lubricants in there; there’s surfactants in there that break the bond the dirt has with that surface.”

Are Touchless Car Washes Better?

So-called “touchless” washes use chemicals and a powerful water spray to remove dirt. Taljan says that automotive finishes are durable enough for any kind of wash, but that poorly maintained bristle or soft-cloth washes could create problems. “As designs, they’re all capable, and I don’t have any hesitation with any of them,” he says. “But as they’re maintained, that’s where you can get a difference.”

For instance, glossy black exterior trim could end up scratched in washes with hard bristles if they “start to fray and get sharp edges.”

If you can, take a look at the brushes before your car goes through the wash. If they look dirty, Taljan warns, they’re probably embedded with dirt from other customers’ cars, which can damage yours.

Dirty cloths or brushes can transfer dirt from one car to another. “You don’t know who came before you, and what was on their car,” Lamberty says, but now it’ll be on your car.

Trouble is, the gentler touchless combination of sprayed-on water and soap alone doesn’t always remove all of the dirt. “You’re relying solely on chemistry,” says Al West, a sales manager at Simoniz, a company that makes chemicals and cleaners for car washes. He says that drivers who usually use touchless washes should occasionally switch it up and get a soft-cloth wash.

Lamberty also suggests that owners carry a soft towel with them to wipe off any residue left behind from a touchless wash.

Do I Need to Get the Undercarriage Wash?

You don’t need this option every time you hit the car wash. But it’s a good idea to opt for an undercarriage wash at least once a season, especially if you drive through mud or live in a part of the country where roads are salted in winter, CR’s experts say. According to Taljan, rust can start when a car’s built-in protective layers are breached, or if mud, sand, and salt get stuck in hard-to-clean recesses of a vehicle’s underbody.

“If it’s down by the wheel well, you might trap moisture, you might trap salt, you might trap something that’s low pH that might be corrosive,” he says.

A high-pressure wash of a car’s undercarriage sprays off those attackers and prevents rust. “Just getting all that stuff off is the focus,” he says. “I like something that cleans the underbody, that’s just getting the junk out.”

Still, some car washes clean undercarriages better than others. “You really have to know the car wash,” he says. “It really comes down to how much water is coming out of those nozzles.”

Should I Pay Extra for Wheel Cleaning?

If you want to get some brake dust off, sure. Brake dust—that black powder that accumulates on rims—has a different chemical composition than road grime and salt, so it requires a different kind of soap to remove it. But unless you’re especially finicky about your car’s appearance, your wheels probably won’t be in trouble if you skip the extra service.

There’s actual science behind using specific cleaners for specific parts of cars. For instance, glass and chrome need a different soap than painted surfaces do, Mosesso says.

Like those other soaps, wheel cleaner is designed for one purpose. “It’s made specifically for steel and aluminum and chrome,” says West.

Over time, brake dust can build up and get caked onto wheels if you don’t clean them. “Would you damage the rim? No,” he says. “But would it make it really difficult to clean it? Yeah.”

Is Spray-On Wax Worth the Expense?

“The finish on a car built anywhere in the last 10 years, frankly, it’s strong, it’s going to last,” says Taljan. While spray-on wax can have some purely cosmetic benefits, he says that a modern car’s finish will last and look good even without frequent waxing, unlike the ’73 Chevy Chevelle he used to drive, which would get chalky without constant care and attention.

Even car-wash owners and soap salespeople agree that spray-on wax adds only cosmetic benefits. “Is it as good as hand-waxing your car? Of course not. It takes 2 minutes,” says West.

More expensive spray waxes—like carnauba wax—last longer. Still, both Mosesso and West say that even a basic silicone-based wax will enhance the vehicle’s existing shine and help prevent water spots during drying.

What About Hand-Washing and Detailing?

Many enthusiasts prefer to have their cars detailed and hand-washed for the ultimate in appearance and paint protection. But for the average consumer, an inexpensive automatic car wash is a good way to go. Even professional detailers see it that way.

“Car washes are meant to maintain a clean car,” says Budhai. “It’s kind of unrealistic to expect that it’s going to scrub it all off and make it perfect.”

According to her, detailing isn’t just about restoring a showroom shine. There are also some messes that an automatic wash just can’t clean. “You can’t get mildew off from an automatic car wash,” says Budhai. “You can’t go off-roading and you’ve got mud stuck onto it and it’s going to clean it all off.” Machines also give the same wash every time. Only a human knows to spend extra time removing bugs from a front bumper or scrubbing tire rims.

CR and other experts warn that hand-washing at home with the wrong materials might do more harm than good. For instance, using the wrong type of soap—Ibbotson says he’s seen folks even use dish detergent—can strip a car’s finish of its protective qualities, and using a sponge improperly may cause swirl marks.

The bottom line, says Jake Fisher, CR’s director of auto testing, is that owners need only keep their cars reasonably clean over time. “Absolutely, owners should make sure the dirt and salt that attack your car’s paint and undercarriage get cleaned off regularly,” he says. “But some of the extras are just for vanity’s sake. You’re better off saving your money.”

Top Car-Washing Strategies

Washing a car is pretty straightforward, right? On the “Consumer 101” TV show, Consumer Reports expert Jen Stockburger reveals the top tips for keeping your ride sparkling clean.

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