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Some people say they wash the outside car every week as it makes the paint nice etc

Are there any benefits of washing the outside of my car?

Do I really need to wash the exterior of my car every week?

Does it damage my car if I don't wash it for a month or two?

I would like a general answer, but the specific car is a 2003 Opel Agila

Obviously I clean the interior ;)

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Do you really need to shower? :) – Zaid Mar 6 at 18:42
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Don't worry about all the negative comments, I religiously wash my car one a year, even if it doesn't need it! – RemarkLima Mar 7 at 7:19
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I'm sure once a week is not required. In a dry climate I can imagine that washing actually increases corrosion (seriously). I find Bob's answer deeply unsatisfying. Of course if you put a wax coat on and religiously inspect your car weekly for spots and fix them it's easy to see how that could be beneficial, like with teeth. But multiply the time spent with your hourly wage and I'm not sure it has an economical benefit any more. – Peter A. Schneider Mar 7 at 14:51
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@Zaid Do you suggest that washing your car often is bad for it the same way as showering often is bad for the skin? Is it good at all (showring and washing)? Is there an optimal frequency or method? Does it depend on climate, city/rural, gravel/asphalt and garage/outdoor parking, or any other parameters? – Peter A. Schneider Mar 7 at 15:43
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Well, if you care about your car looking good in public (they only see external). Also, it really depends on your location & climate. In a country with significant level of air pollution, a car can be covered by dust in just a day. It doesn't look good and it can also damage the material. – Andrew T. Mar 8 at 4:30
up vote 26 down vote accepted

tl;dr: How much did you pay for that shirt? How often do you wash it? Why would you bother? Shirts just get dirty anyway. Can't you get another shirt?

Are there any benefits of washing the outside of my car?

Do you live in an area without salt, birds, tree sap, pollution, abrasive sand or road tar? Do you also drive around with brake pads that don't emit corrosive dust? If so, then you might be okay to avoid washing. If not, then yes, there are plenty of benefits.

Do I really need to wash the exterior of my car every week?

Does it damage my car if I don't wash it for a month or two?

What's practical for your situation? If you can't wash the car for a while because of life circumstances, the point is moot.

Here in the Northeast USA, car washing is actually seriously important. For example, salt is a year-round fact of life here in Rhode Island. In the winter, it's used to treat the roads during snowy weather. In the summer, it's fun to go to the coast (admittedly, Rhode Island is basically all coast but still...) where the wind is picking up all that salt spray and misting it all over the car. Even better, seagulls drop blobs of acidic smears of grossness all over that deliciously salted car.

Some people will try to argue that clearcoat and paint will protect the metal of the car. While that is technically true for a while, neither is sorcery. Any sort of corrosive or abrasive will eventually wear through that protection and start rusting. As a practical example, that gravel shot into the hood of your car by the truck in front of you may have chipped the hood down to the metal. It almost certainly took off the clear coat and a layer of paint.

Any sort of washing will get all that nastiness away from the car with the side benefit of making it look nicer. And remember, anything is better than nothing: I spray out the underside of my car while I'm washing it. I'm not using special undercarriage spray or anything like that, just a straight up garden hose. The startling amount of sand and salt that pours out from under the car could be used to make little castles in my driveway.

It's also a convenient time to just walk around the vehicle and check for any issues. Is that piece of trim coming loose? How long has that screw been missing? Here's a rock chip: I better get some touch-up paint into that right away.

All of the above is bounded by reality. In the winters here, sometimes it's just too cold to wash the car for months at a time. Some of those times, washing the car is effectively moot anyway since all that dirt is getting hidden under layers of snow....

Pro tip: a clean car is much more pleasant to work on. Even if you don't handle your own maintenance, the techs who do will be grateful that they aren't working in a gross muddy rust pit.

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You know, though, I live in the NE USA as well. Salty winters, birds, sap. Aside from the windows, I literally have not washed my car in roughly 9 years (and it's parked outside on city streets); rain and snow wear the worst of it away, and I can't justify washing it since it gets dirty again within a day or two. The car is 15 years old and the only spots of rust on it are in areas around physical damage to the paint. Washing won't keep gravel from busting through the finish, though, and won't keep corrosion from happening on bare spots. – Jason C Mar 6 at 20:12
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@JasonC actually washing will reduce the risk of corrosion because physics and chemistry. If you don't want to wash your car, there's no law that says you have to. Just like your shirt, which likely cost a lot less than your car. – Bob Cross Mar 7 at 0:38
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@Bob I'm not arguing. I just meant that, anecdotally, I can think of at least one case where the finish held up well on an unwashed car. Fwiw. Btw, I'd wash my car every day if it smelled like BO when it was dirty, or if it was always touching my skin, or if the coffee ring on the roof made my hygiene questionable to the opposite sex. – Jason C Mar 7 at 1:27
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@JasonC sounds like an opportunity to do some A-B testing. I have concrete evidence that washing my wife's car pays dividends. In contrast, I'm pretty sure that she would be significantly less impressed if I explained "it'll just get dirty anyway." – Bob Cross Mar 7 at 1:48
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@PeterA.Schneider none of the points in your comment are correct. – Bob Cross Mar 7 at 15:07

The reason many people wash their car every week is to keep it looking nice and from the good old days of bad paint and no rust protection.

These days, paints are significantly better and stand up to some serious abuse, and the cars are much better rust proofed (galvanised etc...).

The washing your shirt analogy is a little unfair as with a dirty shirt, you smell. A dirty car is just a fact of life... More like hiking boots, they're gonna get dirty at some point if you use them as intended. And likewise, to keep them in good condition, give them a good scrub every now and then... But like hiking boots, if you're using them every day then it's easier and more practical to just kick the clods of mud off and start walking again.

A reasonably modern car will be fine only being washed every month or so, if not longer if you don't mind a dirty car.

My personal preference is after the winter months, and there's no more salt on the road, jet wash all the wheel arches, suspension uprights, springs and everywhere you can get to underneath to really get it clean under there, and get all the salt out.

With two small kids, and being self-employed there's not enough time to wash the car every week (let alone every month!), and the interior gets a beating too... But it's a tool to be used, and hence will be getting dirty whilst being used. I've had nice cars, which are nice to keep clean and tidy but then sometimes you just need a workhorse - if the latter, then clean it as often as you want.

Edit to add: I live in the UK where they use tons of salt throughout the winter (we all drive on summer tyres all year round!), so corrosion is a huge problem on older cars - especially 80's and early 90's cars.

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+1 for mentioning the effects of time on tech; compared to Y2K standards, today's cars might as well be covered in "magic armor". With self-healing finishes right around the corner, we might not need to wash them for much longer. – Jon Mar 8 at 15:19
    
Thanks @Jon - I should also mention my car had a paint protection finish from new, I can't recall the name, Duraguard or something similar and to be fair, even chucking some soapy water over the car does make it look like it's been polished! Either way, the days of your racing red being a ladylike pink in the mildest of sunshine are long gone. – RemarkLima Mar 8 at 16:54
    
Totally disagree here. Ever seen 2009 cars of same brand and color with similar milleage but one having great paint condition and clear lights VS other having yellowed out lights and very dull paint? That's what happens when you don't wash for months. – Alexus Apr 25 at 22:03
    
Is that more down to UV exposure? One garaged and one not? From the yellowing lights that's what it sounds like... My car is 2010, kept outside, rarely washed but when it is, the paint is absolutely fine. Sounds like something else going on there, more severe than just not washing... – RemarkLima Apr 26 at 6:01

I work from home, so my car stays in the garage for long periods of time. It doesn't get much mud or salt spray, and after it's washed, it looks pretty fresh for a month or two. It's often 6-months to a year between washings, and it never really looks terribly dirty (If it does get really dirty, I will wash it). However, a couple of years back, when I went to finally wash it, some bird droppings had been there for a few months, and had eaten through the paintwork, leaving whitish patches. Now, I still don't wash my car a lot, but if I notice any droppings, tree sap, etc on it, I will spot clean them off right away. Any leaf build up at the windscreen or boot openings should also not be ignored, so it doesn't hold moisture.

If you care about long-term damage, even if the car is pretty clean, some things still need attention.

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I wash my car before winter every year before it gets too cold. I give it a thorough detailing and then I apply 2 coats of hard wax to make sure I did a good job. This is my idea of giving the body some protection and it seems to work. After all it is a significant financial investment for most of us. You'll notice the water not sticking to the paint work at all for some time. I don't do it out of aesthetics or anything like that since consuming water for such things bothers me.

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I wash my car when it gets so dirty I can't walk past without my clothes getting dirty. This is really about every three months, with maybe a special exception after visiting the sawmill or so. At the start of winter I have it washed with underwash, and hot-waxed. At the end of winter I do it again. Being a lazy engineer I have it done at the carwash. The spousal unit like to do it himself, which keeps him occupied I suppose, but in that time he could be e.g. chopping wood (I've been married long enough to keep that thought to myself). – RedSonja Mar 7 at 14:57
    
The water is rainwater out of the cistern and the electricity is solar, so maybe his conscience is cleaner than mine? On the other hand, the carwash catches and cleans the water, while here it just runs into the lawn. – RedSonja Mar 7 at 14:59
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@RedSonja I drive my car onto the front lawn to wash it. That's the only water that our lawn gets in some of these hot summers. Even better, my soapy water is getting filtered through my grass, plants and dirt rather than hitting the storm sewer directly (which are only a hop or two away from our local ocean). Pro tip: summer roasted grass doesn't care about soap at all. It's just so glad to get any water at all. :-) – Bob Cross Mar 7 at 15:14

Apart from the obvious benefit that a clean car looks nicer, washing your car will also remove all sorts of stuff that, on the long run, will harm its exterior: Salt attracts moisture, which speeds up corrosion (which would not be much of an issue with a galvanized body and intact paint and clearcoat, but a 13 year-old car is likely to have some faults in that corrosion protection). Bird excrements, insects, sap from trees and other kinds of dirt may be chemically aggressive and damage your paint – the longer the exposure, the greater the harm.

There is no hard-and-fast rule how often you should wash your car – it depends on the kind and quantity of dirt your car is exposed to. A car that is used every day in winter conditions and is usually parked outside next to a busy road will obviously need more care than a car that sits in the garage for most of the time and only ever gets taken out on weekends and only in sunny weather.

That being said, washing is not the whole story. Exposure to sunlight will eventually render the clearcoat porous (on some older, poorly maintained cars you can actually see it peel off), causing moisture and dirt to "stick". Waxing your car every now and then will restore the original properties of the clearcoat. It will stay water-repellent and slightly elastic, which prevents it from cracking and peeling off.

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I have a 2004 Subaru Forester and have had it since it was about a year old. I can probably count the number of times I have washed it on one hand. They wash it when the oil gets changed at the shop but I don't drive much so that isn't very often either. It's pretty dry here but we get lots of snow in winter. They use gravel but not salt on the roads. I just recently had to polish the clear plastic over one headlight because it made the light dim in comparison to the other side that had that piece replaced. Other than that it doesn't seem to have mattered. I'm sure I've been judged for it at some point but I don't care. :)

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I assume that if you ask "Are there any benefits of washing the outside of my car?" you are not referring to the obvious cosmetic benefits. So I assume that the overall question is essentially summed up by the sub-question

Does it damage my car if I don't wash it for a month or two?

There is not much fact based evidence for an answer, with one exception: The corrosive effects of salt in combination with moisture on steel. This pdf from the Transport Research Board links the combination of road salt and acid rain to vehicle corrosion in the 40 years after WW2. Mostly in the 1960s, road salt usage and acid rain increased to form a corrosive mixture which forced the auto industry to respond by improving body design and materials. In the 1980s this resulted in cars which are far less prone to corrosion than those from the 1960s and 70s, if the paper can be believed.

There are two main sources for salt. Obviously in coastal areas sea water spray transports salt, together with moisture, through the air.

In places with snow and ice deicing salt is an even greater danger. It is picked up from the street surface by tires (your own or others') and transformed into a salt spray around busy streets. The salt concentration is higher than with sea water spray.

Common sense dictates that removing the salt prevents corrosion and that a protective coating keeps the salt from contacting the surface in the first place. Note that the places most in need for cleaning and coating are not the ones with the greatest cosmetic effect, because they mostly face down: The underbody including pipes and mechanical elements, the interior of the fenders, and perhaps the lower door edges and similar painted surfaces.

My common sense lets me think that with ongoing salt exposure the washing alone will have little effect on a car which is used often: the next ride will re-salt the clean surfaces, essentially un-doing the effort. It's the waxing/coating which has the long-lasting effect. There is an consensus that waxing should be done at least twice a year. I would think that a seasonal cleaning when the salt period is over is a good idea as well, coating or not. —

For an area without salt plus moisture the benefits of car cleaning are much less clear. Reliable, substantiated information is hard to come by. One of the problems with web sites making recommendations is that most of them fall in one of two categories: Amateurs, often forums; or professionals with an economic interest in attracting customers and selling car care products. Some sound more reasonable than others, but none of them cites any reference; one must suspect it's all anecdotal evidence at best, or hearsay and copying from elsewhere.—

Anecdotal evidence suggests that in a desert car bodies last long no matter what you do to them. (Image: A Mexican pulling you out of the sand with his '72 Chevy.) If you live in Salt Lake City washing may actually add the moisture to the salt which starts the corrosion; who knows. I'd be interested to hear from people living there.

Since you have an Opel, I checked their (German) web site for advice about care and maintenance. They are very brief and just say "a regular car wash protects paint and the car in general from rust damage". They do not recommend a specific frequency. Interestingly they warn about damage from bad car washes: "Spend a few Euros more and patronize a modern facility; it's worth it." That is an interesting point: Car washing done wrong can actually damage your paint. Increasing the frequency will then just damage it more.

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As someone who lives in (well, near) a desert, I can confirm your suspicions: cars don't rust around here without serious provocation. – Mark Mar 8 at 3:34

I leave in NJ, and have been driving my volvo for 15 years now. I only wash it once or twice a year when I get free carwash coupons. Not a speck of rust on the exterior.

Washing it after Noreasters makes little sense since I would have to drive back from a carwash through the same horrendously dirty roads. Washing it after the winter makes even less sense - spring rains do the same job free and without harmful chemicals.

From what I see around me, people have similar attitude, and there are very few rusty cars around.

You can not make a meaningful comparison with modern humans though - they are not exposed to rains as much as the cars do.

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Washing it when the roads are muddy/salty actually makes more sense. You can get that crud out of the places it has been forced since the previous wash, and a wax coat helps it stay out. – Rory Alsop Mar 8 at 8:07

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