I don't have enough time to answer you fully right now, but I'll try something,
Glass Coating like they call it overseas is a coating containing something related to glass / silica in it's constitution.
I'll not go deep into this subject since my chemistry wouldn't be enough.
In Brazil, they call coatings by 'Vitrificação', comes from 'Vidro' which means Glass. The word for coating in Portuguese / Brazilian would sounds in english something like:
'Glassification' of a car finish: application of a protection product based on glass / silica components, and related.
Those coatings will 'harden' after cure, and it's likely to add some thickness to surface (again, cannot cite an exact number). This will vary greatly depending on product and application method / cure, etc.
In my opinion they can be layered, although I don't have a definitive answer about the best 'timing' for applying overlays, I'm moving my sticks and doing some tests.
Now, to what extent they can be layered and this will improve it's thickness / protection, if this is good, if this will interfere with curing, there's no exact answer. Will it improve looks?
What you're doing right now if not applying 4-5 coats in your own test vehicle?! Please, share and let us know your results. You already stated, they are 'relatively cheap'.
Only people really involved into development of those products can answer the above questions, but maybe answering all questions would be not interesting for them.
There should be more collaboration of coating manufactory with actual detailers to fulfill actual needs about the product.
Recently I've researched and was able to put my hands on those Glass Coatings. I've already started to apply it on test subjects, with great results.
- you're right, 20ml of a 'glass coating' are enough for ~2 cars, and exactly 2 coats on 1 car, very well applied.
I've liked the results, and based on other products I've tested to date they are very similar to CQuartz. They are definitively thicker (product consistency) than Pinnacle Black Label Coating, which I may use interchangeable with it on a coating package. When they dry on applicator / towel, you feel it has hardened, like CQ.
Both has it's strengths and weakness, both are excellent products, maybe on top with the best we have nowadays in terms of Stunning final results.
If you use a polish, you'll be polishing the coating layer. Depending on how it's attached, a cleansing / non-abrasive product may give your coating a refresh without abrading it. If you're using abrasives, I believe you'll start removing some coating until it's done, if using a wool pad and aggressive compound this may happen really fast. Everything, depends.
What I also have as results from my tests is that after polishing over a coating, you can do an IPA wipe on section worked and reapply the coating, if you wish to the whole panel, or whatever fits the situation to get the best from the job you're offering.
Another 'tricky' part about those coatings I've noticed is that sometimes a damped microfiber towel (or even IPA) is needed to remove it's residue towards achieving a streak-free surface.
According to my tests and applications, IPA mix for removing residue worked very well after ~7-10 min of product evenly spread then to a haze.
Although I've read 'throughout the world' people complaining about IPA use just after 'LSP' application, in terms it would 'weaken' some aspect of the LSP, but let me tell you in the 'Real world' I've not faced this. After hazing, IPA wipe will not remove glass coating (not to any noticeable level, at least). It's a coating, not a layer of carnauba wax.
Removal with only a dry, clean MF will probably ends up 'linting', and suede towel will spread the haze here and there ahhaha
After the damped MF, you can now use your clean / dry MF for the final buffing without any lint issue, and become amazed with surface slickness like you rarely (or never) seen before...
Please, let me know if that helps or if I've deviated the subject (ahahha), tried to put some ideas on table so we can talk about what I believe will be the 'future-present' of coatings the coming year.
I'm also here to hear more about who's also using those coatings.
Kind Regards.