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      05-20-2011, 04:26 PM   #1
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Power Washer PSI

I'm shopping a pressure washer to apply foam presoak to my car. I'd like to use it for washing as well.

How much pressure can I safely use on the paint & gelcoat?
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      05-20-2011, 10:14 PM   #2
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I heard 4000psi can rip the siding off the house

As far as I know 1100-1300psi is the max to use on wheels/paint
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      05-21-2011, 12:55 AM   #3
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I gave my 2 cents on this topic a while back in another thread posted above

You don't need high pressure, you need water volume - gpm (gallons per minute)

Modulating pressure through tip selection utilizing interchangeable quick release tips with varying pressure ratings can turn any power-washer in to a much more versatile tool. Water volume remains constant (this is the job of the pump) while the pressure is regulated (up or down depending on the diameter of the opening of the tip) by the restriction that the tip provides up to the max pressure point of the pump. Most pumps are rated beyond the rated pressure for the machine and can be run above that, although it adds wear and tear to the pump.

There is a simple calculation used for "cleaning units" which is a loose measurement of cleaning power of a set up.

pressure x gpm = cleaning units


3000 x 3 gpm = 9000 cleaning units

2000 x 5 gpm = 10000 cleaning units

So - the 2000 psi 5 gpm machine is actually more powerful than the 3000 psi unit

You also don't want to use a power-washer to apply soap or foam. You end up using so much water (because the gpm is so high at 2-3-4 gpm) that the soap is so diluted that it's ineffective, or you'll use way too much soap and 80% will end up on the ground. Buy a standard garden sprayer and premix the soap in it then apply with a fan or cone tip.

I would suggest a 2500 psi machine at 3-4 gpm and utilize larger tips that bring the pressure down to 1500. If you buy a machine that's only 1500 psi the functional gpm will be too low for varied use.
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      05-21-2011, 08:37 AM   #4
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I've been using a PW on my car for the past two years without any issues.
(Not with every wash but once a month).

I have a gas 2600psi (2.3gpm) PW (I would not recommend my PW itself due to problems with reliability)

Like mentioned above, the psi is "controlled" various ways (tips, distance). I never get the tip close to the paint surface. Usually its atleast a foot or so away (maybe the wheels are an exception to this). Its priceless for getting brake dust off.

I pair it with a foam cannon, and find it really minimizes swirls before hand bucket washing, especially if really dirty.

Here's a good discussion by some pros regarding PW:

Hope it helps.
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      05-21-2011, 09:35 AM   #5
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as mentioned it depends on the tips you use & distance from the surface. I have taken paint down to metal with my pressure washer & was only using the third most powerful tip.
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      05-21-2011, 11:46 AM   #6
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Just picked up a Greenworks 1700 psi(~$190), seems a tiny bit lacking in power, but I probably wouldn't go above 2200. Using it with a nice $20 foam cannon and the results are pretty nice for how cheap the set up was, and I didn't even use a dedicated foam soap yet. Also, one of the $100 foam cannons would likely work better. Here's a pic from the cheap set-up of an E46 M3 I detailed a couple days ago:

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