Most of the fine swirl marks you see in car paint under direct sunlight didn’t come from the road. They came from washing. The good news is that avoiding them is mostly about the order you do things in and what you use.
Start in the shade
Washing a hot panel in direct sun means water and detergent dry before you can rinse them off, leaving spots and streaks. Early morning or late afternoon in the shade is ideal, particularly through a Perth summer.
Rinse before you touch anything
The single biggest cause of scratching is dragging grit across the paint. Give the whole car a thorough rinse first to float off loose dirt and sand before a wash mitt ever touches a panel.
Use two buckets
One bucket holds your soapy water, the other holds clean rinse water. Rinse the mitt in the second bucket after every panel so you’re not putting the grit you just removed straight back onto the paint. A grit guard in the bottom of each bucket helps.
Work top to bottom
The lower sections of a car — sills, bumpers, behind the wheels — are always the dirtiest. Wash the roof, glass and upper panels first and leave the bottom until last so you’re not spreading the worst of the muck upwards.
Wheels last, with separate gear
Brake dust is abrasive and it will wreck your paint mitt. Keep a dedicated bucket, brush and mitt for wheels, and never use them anywhere else on the car.
Dry properly
Letting a car air dry leaves mineral spots, especially with bore water. Use a clean microfibre drying towel and pat rather than drag. Open the doors and boot and wipe the shuts while you’re there.
A quick note on automatic car washes: the brush-style ones are the most likely to mark paint. Touchless washes are gentler, though they won’t shift heavy grime as well.