Lots of opinions here, but I think it's important to keep an open mind.
Those who support a Grit Guard make the following assumptions:
1. Dirt is scraped off the wash mitt by the guard (why else would they now make a washboard version?)
2. Dirt falls to the bottom of the bucket due to gravity
3. Dirt stays at the bottom and doesn't ascend during agitation of the water and because of the panels of the Grit Guard
4. Therefore, the actual wash/water coming in contact with a sponge/mitt is less contaminated and, therefore, one is less likely to instill scratches or swirls on the paint.
Proponents also argue:
1. Grit Guards are inexpensive, so why not use one?
I use two Grit Guards (wash and rinse buckets) because they are cheap.
As for my take on the issue of whether they help or not:
1. Show me the data.
I won't be apologetic about it since I am a scientist and an academician. There are lots of things that in theory seem to be reasonable conclusions based upon the purpose of a process. One does not need a randomized placebo controlled trial to see if parachutes save lives when jumping out of an airplane, but in most other cases, you really need some good data to run with an assertion that some method is superior to another.
Grit Guards may be proxies for people who are already meticulous with car washing. Most of us already have pretty swirl-free cars, use QD when needed, have great LSP on our cars and wash as often as it's prudent. The fact of the matter is that solids do suspend in solution, and not all of this will descend to the bottom of the bucket. The act of plunging a sponge/mitt into the medium will cause agitation of the solution and disruption of any solids on the bottom of the bucket.
For those that use 2 buckets or 3 buckets, how can you not account for the variable of rinsing 1 or 2 times? Maybe that is what frees the dirt from the mitt and saves the paint. What about those who use 1 or 2 rinse buckets and 2 or more wash mitts? Maybe it's the detergent in the wash and rinse buckets that create an immiscible solid and it doesn't get back into the MF. I alternate wash mitts when I wash my car. Maybe it's the alternation of wash mitts and the extra soaking/rinsing that saves the paint and not the grit guard. Maybe it's all components combined. Maybe it's the fact that my car is pretty clean in general and that it's coated that saves the paint when I wash it. Maybe my wash technique is great. Maybe my wash technique sucks but all these other things compensate for my technique. What about foaming before or soaking with ONR before a wash? Shouldn't that help too?
My point is that the a priori conclusion that Grit Guards work because they look like they work and because we see dirt at the bottom of a bucket is unfounded. I'm sure there are plenty of people (as demonstrated here) that don't use them and have swirl-free cars. But, there are probably multiple variables that go into washing and maintaining a car.
Of course, a simple response will be: every little bit helps. Possibly. But how much?
Do Grit Guards worsen the outcome? Probably not, but I have no data to support that either.
As for now, I'll continue to use them because they make sense -- generally speaking. But I do a lot of things that make sense when in reality there's no proof that it actually makes a difference.
Another thought: If you plan on doing a full correction of a car that already is heavily swirled, does it matter in this situation especially?
(BTW, I just bought the washboard.)
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