P&S, the same people that make Beadmaker sealant, offer one of the best microfiber detergents on the market. It's called P&S Double Black Rags to Riches. It not only cleans really well, it can actually restore the towel's absorbency, color, and softness.
P&S Double Black Rags to Riches Detergent - The next generation of microfiber cleaning technology! Use P&S Double Black Rags to Riches to get a newly softened, brightened look and feel to your favorite microfiber towels. P&S Double Black Rags to ...
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I also find that if you pretreat the dirtiest sections of the car prior to an ONR wash using a garden sprayer filled with ONR solution, a lot of the dirt will run off the car, leaving less dirt to be picked up by your towels.