When you say "#20 over #21 and SG ". I'll take that as meaning , which should I pick to use out of the 3 . Basically all of these products are sealants , #20 is a Polymer Sealant , #21 is a Synthetic Polymer Sealant and Klasse SG is a Synthetic Sealant . Synthetic's seems to be real popular today both in waxes and sealants due to their durability over regular wax and ploymer sealants. Confused yet ! just kidding, I like SG and #21 you can't go wrong with any of the 3 you listed . I'm sure someone else will chime in .
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