craigdt- Was the UK forum guy the person who "proved" (scare-quotes utterly intentional!) that Collinite "doesn't layer" (again intentional)?
Heh heh, it takes some pretty high-falutin' equipment to measure build-films that're *that* thin..just measuring microns takes a pretty decent ETG and often shows how much even factory paint thickness varies. Whole lot simpler/easier/cheaper, and IMO more productive, to just do your own LSP Layering Test on some representative panel IMO and see if the multiple applications make a diff for *you*. A huge part of my opinion on that is the way there are a few variables in play (application method, time between applications, and probably some others that I'm not thinking of) that can utterly skew the results depending on how the test is done.
It's subjective in the sense that different subjects can/will achieve different results.
Classic example is the layering of of Autoglym Super Resin Polish. It's an AIO that contains both (very) mild abrasives and chemical cleaners. "Shouldn't layer" according to the Conventional Wisdom on the subject. But it absolutely *CAN* be layered IF (awfully big "if" in this case) you go about it right.
(Noting that this is the Griot's Forum, my initial introduction to AutoGlym SRP was via one of Richard's earliest catalogs. Bought it from GG until they quit offering it.)