To the best of my knowledge a glaze does not effect the longevity of anything,, glazes will change over all appearances, for example it will give a dark color a depth that is like no other, on the lighter colors it gives an added deep wet shine to the over all finish, its almost like light refract between the between the 2 layers. (the Glaze and LSP)
as far as how much to use?
Black Hole and White diamond both remove easily even with a slightly heavy application. if you do decide to use BH or WD and apply your LSP with a machine use the possible LOWEST setting,, Glazes do not protect and should not be used as an LSP, there soft and do not take to being worked to much after setting up, so my best advice is to apply your LSP by hand so not to lift the glaze.
My personal preference is to use BH on darker colors and WD on lighter, as of yet I have not done extensive side by side comparison with BH and WD on the same color.
Hope this helped.
Good luck.
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A Detailed finish is like porn to most of us.