Yes, it's recommended that you wet your car before using a foam cannon. Wetting your car beforehand helps to loosen and remove any loose dirt or debris that may be on the surface, making the cleaning process more effective. Additionally, it helps to prevent any potential scratches or damage that could occur from the foam cannon's high-pressure stream hitting dry dirt or debris on the surface.
To use a foam cannon effectively, simply rinse your car with water to remove any loose dirt or debris, then apply the foam using the foam cannon. The foam will cling to the surface of your car and begin to break down any dirt and grime. After allowing the foam to dwell for a few minutes, rinse it off with water, and then proceed with your regular car washing routine.
It's important to note that different foam cannons may have slightly different instructions, so it's always best to consult the manufacturer's instructions for your specific foam cannon model.