How To Use A Foam Cannon. Many people use foam cannons, And they don't even understand the purpose. Using a foam cannon correctly reduces your opportunity to scratch and marr the paint. foaming first allows that initial debris to run down the paint and capsulated by the foam and to fall safely on the ground so that when you rinse you're not embedding all that dirt and sediment deep into the paint. once you put on that next set of foam that is actually for the lubrication you need to do your cont