Great question my Friend, under normal circumstances we would use our paint gauges but once a car has been re painted this rely renders them useless because with multi layers of paint its impossible to work out how many microns of clear cote are on the car, as was the case on this car, this is when having years of experience with automotive paints from my restoration and show prep business, my experience goes back to when cars where painted with cellulose paints and I still work on collections that include this paint today, so getting back on track, we first inspect the imperfections that sit in the paint and try to determine if they would either flat down level or flat out completely this said that there is no guarantee that you will not flat or polish through { I do have a video showing what can happen when you chase a mirror finish }, in most cases the owner of the car is in the position where he is advised that the car would rely need a full respray, he would then have to decide if he would want to take the chance at trying to rescue what is already on the car and understand that there is always the possibility of going through the paint.
This car had been built rely well and would justify having a good re spray, but quality resprays are very labour intensive and so very expensive, so the cars owner decided that it we would have a go at making the car look reasonable, it was explained that it would never be any where near a perfect finished car but I felt that I could probably get it looking reasonable but would never be to the level that I could normally achieve but at the same time I relished the challenge that the car presented giving an opportunity to use skills that lie dormant for most of the cars that I work on.
The second part of your question concerning laying loads of paint on the car so that it can be worked back to a finish, my opinion is, that if a body shop is using this method, you would have to ask the question, is the other parts of the repair any good, anyone who works using this mindset, should not be painting a car, there is a different mindset when painting a car, if you are working on a wing that has to be a close finish match to the adjoining panel, a good painter would know how to set his gun and air pressure also technique to get a finish close to a match knowing that a little polishing between these panels would be very close , if you where painting a car that is going to be a show car and needed extra Coates of clear cote, it would not be just put on thicker in less coats but would have 5 or more controlled Coates of clear cote applied and not puddled on {this would leave a very un natural finish when polished} and believe it or not having an orange peel finish does not necessarily mean there is lots of room for flatting and polishing.
Having lots of orange peel on todays factory painted cars means paint shops and detailers have to be very careful during flatting and polishing these finishes and paint thickness will continuously be measured during the flatting and polishing stages, this is because the manufacturers are trying to build cars without wastage of product, they know exactly how many grams of paint and lacquer are needed to adequately cover a car and are not going to give much more than needed. I hope this helps to answer your question, but trying to type an answer, rather than stand in a paint shop where we could actually show you is quit difficult but I hope I have given you some idea of our thinking, when we are facing a panel that needs some love.