The first thing to consider is that all wax and sealant coatings are microscopically thin. How thin? So much they cannot really be measured with traditional methods but must be applied to special glass discs and tested for light refraction. Ultimately we are talking sub micron levels of thickness or about .000018 on an inch... (3/10ths of a micron)
Yes, testing has been done to confirm "layers". In most cases, not all, both paint sealants and waxes exhibit maximum thickness after 2 coats. In many cases, 2 coats produces a thickness that is more than twice the first coat. This universally has fallen apart after the second coat. In some cases a third coat will product almost no change in thickness (slight +- variances aside). Usually we find that a third coat will actually begin to reduce the thickness of the coating, likely because of solvent and friction.
Now we are talking about layering two identical chemicals on a glass disc. When we factor in the fact that we may experience a more permanent bond to paint (vs. glass) and product curing should be more complete on paint (vs. glass) then it becomes feasible that multiple coats have a greater benefit.
Testing, using feeling, looks, water beading contact angles, etc, can prove that some products stick on top of others and some do not.
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