Clay bars are used to remove organic and metal contaminates in the surface. In layman terms they make your surface rough and gritty.
Glass does not need a special clay bar, and most clay bars are safe for glass. The only bars I would not recommend for glass are "heavy" or "aggressive" clay bars. They may leave marring on the glass and need to be polished afterwards.
With any clay bar, make sure to use the proper clay lubricant. Soaps and water-less washes can often breakdown the clay bars, causing them to crumble and fall apart.
When the claying is complete, you need to protect the glass with a sealant. This will prevent the glass from yellowing, and help protect it from external contaminates.
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