Dan said:
Mainly because not a single person has been able to show that there is any sort of benefit. In fact there was a great study posted on another forum where the author used expensive equipment to prove that after the second coat, the film thickness actually decreased.
-AND-
TOGWT said:
Layering is more about appearance (depth and etc, and is very subjective) than durability; twice as much won’t last twice as long.
IME it just depends on the product and the application method.
I too saw that layering test with the Collinite (what'd he use again...

.... I think 915) and thought that a) he should've waited longer between coats, and b) he shoulda spit-shined it if he was gonna recoat so quickly. *IIRC* and maybe I don't, he just did immediate subsequent coats applied in the "normal" way.
When I experimented to see which of my LSPs were worth layering and which were not, and also what application methods were required to mitigate all the solvent-effect factors, I found that multiple coats of Collinite 476S and especially 845, *done properly*, lasted months longer than a single coat. Night-and-day, no way to misconstrue type difference with the layered side "still waxed" and the single-coat side "dead".
With Sӧuveran, OTOH, I had to do all sorts of spit-shine/hassle/PIA stuff to get any difference, so I decided it basically doesn't layer and it's best to just to a coat after every wash.
Waxes developed for mold-release applications (M16, FK1000P) simply *must* layer to work for their original purposes and thus will layer on cars. But not always without cosmetic issues (the pseudo-hologram effect one can get with the FK).
Easy enough to test for layering, just equalize the test period's starting times between the single/multiple layer sections (something I haven't seen people do in layering tests).
If you put a more durable product on top of a less durable, you get the durability of the base. So in the end, it is a waste of time..
Again, IME it just depends (probably on the specific products being use). On most of my vehicles I apply durable LSPs (FK1000P, Collinite) over not-durable bases (AIOs, 1Z WPS, 1Z Pro MP), base layers that won't last a month by themselves. The vehicles don't need re-LSPed any sooner than when I apply those LSPs over bare paint.
Best example of this is probably my '93 Audi, which has the same LSP regimen (Collinite) on the whole car, but a mix of bare-paint panels and othe panels with the 1Z WPS. Zero diffs in how long the LSP lasts even though the WPS is lucky to last more than a couple weeks by itself.