The Two-Bucket Method
Quick answer: The two-bucket method is a washing technique that uses one bucket of clean soapy water and a separate one to rinse your mitt, so grit ends up in the rinse bucket instead of being dragged back across the paint.
What it means
You wash from the soapy bucket, then rinse the dirt out of your wash mitt in the second bucket before you reload with suds. Grit guards sit in the bottom of each bucket: plastic grilles that let debris fall through and keep it down there when you agitate the mitt above them. The point of the whole arrangement is to break the loop in which one particle of road dirt goes back onto the panel again and again.
Sequence matters as much as kit. Panels are washed from the top down, because sills and lower quarters carry the coarsest grit, and a mitt that has been near a sill should be rinsed hard or set aside entirely before it goes anywhere near a bonnet. Most people use a single mitt for the whole car, which is where the method quietly stops working.
Why it matters
Most swirl marks are put there by washing, not by driving. Every fine scratch you can see in a bonnet under low sun was dragged across it by something, and in the great majority of cases that something was a dirty mitt, a sponge, or a brush at a hand car wash. Keeping grit away from the paint is the cheapest protection there is.
On a freshly polished or coated car the stakes rise. Paintwork correction removes a measured amount of clear coat and none of it comes back. A car washed carelessly for a year undoes the work; the same car washed with two buckets and a clean mitt still reads as corrected three years later. That gap is the largest single variable between the cars that come back looking good and the ones that do not.
Where it goes wrong in practice
The principle is sound and the failures are nearly always in the detail. Rinse water gets reused until it is as dirty as the wash bucket. Grit guards get bought and then never really used, because agitating a mitt properly takes longer than dunking it. The mitt goes on the driveway and comes straight back up. Cheap microfibre sheds, then holds grit instead of releasing it, so a good mitt is not vanity: it is the only part that touches your paint.
There is also some honest arithmetic. Two buckets, two grit guards, a decent mitt, a pH-neutral shampoo, a pressure washer for the pre-rinse and a few drying towels add up before a drop of water has moved, and a careful two-bucket wash on a family estate takes the better part of an hour. Done properly it works exactly as described. Done in twenty minutes with one bucket and a household sponge, it is worse than leaving the car dirty.
Context
Car washing is best understood as a ladder of contact. At the top is touchless: pre-rinse, snow foam and a pressure washer, where nothing solid meets the paint at all. Below that sits the two-bucket method, the standard for safe contact washing. Below that come single-bucket washing, sponges, and the rotating brushes of an automatic wash. Every step down the ladder trades time for scratches. Two buckets is simply the point at which contact washing stops doing measurable harm, which is why it has outlasted every product fashion that has come and gone around it.