Google usplastic and in their search box type grit, purchase two, one for the wheels, tires, wheel wells, use the other grit guard for the finish.
Your cars finish will love you for this dedicated system.
Al
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The way a person treats their car is a direct reflection of how they treat themselves
I got the one from Meguairs. It really works, the grit and sludge stays at the bottom. I can no longer find it on Meguiars website but the original (which is identical to mine) is available here for under $10
The concept is to keep the swirling action caused by the mitt being dunked into the bucket from stirring up dirt and redepositing it on the mitt when you pull it out. The grit guard has baffles that keep the water from swirling around at the bottom. I don't know if just using something that keeps you mitt off the bottom would do any good, afterall, what's the mitt doing at the bottom, mine floats.