Quick answer
The aim of any exterior wash is to remove dirt like mud, insects and bird droppings, and to remove contamination like brake dust buildup, tar, tree sap and airborne pollutants, without damaging the paint. A freshly detailed car looks better, feels better, and can even sell faster or at a higher price.
Why Washing Can Scratch the Paint
Car paint is a sandwich of layers: the metal panel, a base coat (a type of primer), the paint that gives the car its color, and the clear coat on top. The clear coat is built to be UV-resistant and tough, and it is the layer where the damage happens.
A scratch is a groove in the clear coat, created when an abrasive particle contacts the surface under pressure. With sufficient lubrication, cleaning a surface is safe. Without it, abrasion takes over and two surfaces in contact start grinding against each other, which is where scratches and swirl marks come from, even from tools and towels that are perfectly clean.
What You Need
- Two buckets with grit guards. One for the water-and-shampoo mix, one with clean water for rinsing your mitt. Each grit guard sits at the bottom of a bucket and filters dirt out of the water.
- A wash mitt.
- Car wash shampoo.
- A pressure washer.
- Microfiber towels for drying.
- Wheel tools: a barrel brush, a long bristle brush, a wheel wash mitt, and a bucket of water with a dirt trap.
Before you start: don't work in direct sunlight, and wait for the car to cool off. Avoid surfaces that are extremely hot or cold. Keep the pressure washer nozzle at least 20 cm (about 8 inches) from whatever you're washing, and remove the license plates and license plate holders.
The Correct Washing Order
1. Wheels and rims first.
Spray iron remover directly onto the alloy wheels, working top to bottom. Wait for the car to cool off first. Wash the rim barrel with a barrel brush or a wash mitt, use a wheel wash mitt on the face of the rims, and use a longer brush for the screws and bolts. If one rim takes longer than the others, the chemical can start drying on the rims you haven't gotten to yet. Re-apply it, or spray a little water from the pressure washer to keep them wet, so the dirt doesn't resettle.
2. The pre-wash (contactless).
Before any physical contact, remove as much dirt as possible. Start with a pressure wash, paying attention to the areas that collect the most dirt: panel gaps, door handles, emblems and grilles. Apply a contactless foam, let it sit for a few minutes to soften the top layer of dirt, then rinse the car again with the pressure washer. If the car hasn't been washed in a while, an optional step is to go over the same problem areas with an all-purpose cleaner and a soft bristle brush, then rinse again.
3. The two-bucket hand wash, top to bottom.
Fill both buckets with around 10 liters (about 2.6 gallons) of water, and put a grit guard in each. In one, add around 50 ml (about 1.7 fluid ounces) of car wash shampoo and mix it with the pressure washer. Dip your mitt into the shampoo bucket and start with the roof, since the dirt there tends to be the least abrasive. Move the mitt left and right, covering about a quarter of the roof before flipping the mitt to a clean side and covering the next quarter.
Once half the roof is done, dip the mitt into the clean rinse bucket, shake it a little to release the dirt, squeeze it out, then dip it back into the shampoo bucket. Repeat this pattern for every panel: shampoo, wash a section, rinse the mitt, shampoo again. Always work top to bottom, and save the lowest, dirtiest parts of the car for last. For those sections, consider using an older microfiber towel instead of your good wash mitt. When you are done, rinse off the soap and dirt with the pressure washer.
Do: flip your mitt to a clean side often. Don't: press hard. Let the chemicals do the lifting, not your arm.
4. Drying.
Dry the car top to bottom with a towel, in linear motions, and use a microfiber towel for specific areas and the rims.
When to Do a Full Decontamination Wash
Run your hand across the "clean" paint. If it feels rough, almost like there are a thousand tiny dots under your fingers, that is contamination buildup: a mix of iron particles from brake dust, tar from asphalt roads, tree sap, pollen and other airborne chemicals. For a full detail, or before polishing, you want a full decontamination wash.
- Chemical decontamination: spray iron remover across the whole car, let it sit for a few minutes while you watch for its distinctive reaction, then rinse. Use a tar remover on the tar spots.
- Physical decontamination: use a clay bar with a dedicated clay lubricant, on a wet surface.
Details are in Clay Bar vs Chemical Decontamination.
If you are washing before polishing, or the car is already covered in swirls, you can skip a few precautions. There is no need for the two-bucket system, and foam is only needed if the car is extremely dirty.
Want to master exterior care?
My Exterior Car Detailing Course covers washing, decontamination and protection, in order. It has 11 lessons and a quiz on Udemy.