Snow foam

Snow foam is a thick, clinging foam sprayed onto a dry or damp car before any mitt touches the paint. It is applied using either a foam cannon connected to a pressure washer or a smaller foam gun that hooks up to a garden hose, and it is left to dwell for a few minutes so it can soften and loosen dirt on its own.

The point of snow foam is to do as much of the cleaning as possible without physical contact, since dragging a mitt across dry, gritty dirt is one of the biggest causes of swirl marks. By the time you rinse the foam off and move to a contact wash, much of the loose grime and road film is already gone, so the mitt has far less abrasive material left to push around.

Foam strength is controlled by the dilution ratio in the cannon's bottle, and thicker, longer dwelling foam generally needs a stronger mix on very dirty cars. Look at whether a cannon needs a specific pressure washer flow rate to foam well, since underpowered washers often produce a thin, runny foam that slides off before it can work. A common mistake is treating snow foam as a full wash on its own and skipping the contact wash entirely, which leaves embedded contamination behind.