The short answer
Rinse the whole car first, then use two buckets, one with shampoo and one with plain water to rinse the mitt in between panels. Work top down and never in direct sun.
- Rinse the whole car thoroughly first to float the loose grit off before anything touches the paint.
- Two buckets: one with car shampoo, one with plain water. Grit guards in the bottom of both if you have them.
- Rinse the mitt in the plain bucket after every panel, then reload with shampoo.
- Work top down, so dirt runs onto panels you have not done yet.
- Do the wheels last, with separate tools that never touch the paint.
- Dry with a large microfibre drying towel, patting rather than dragging.
What will ruin it
Never use a household sponge or a dish cloth. A flat sponge traps grit against the paint and drags it across the surface. Never use washing-up liquid either: it strips wax and any sealant.
Why it works
Swirl marks are thousands of fine scratches from grit dragged across the clear coat. The two-bucket method exists for one reason: to stop grit going back onto the car from the mitt.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.