Wow, take a step back there, guy. We're not actually bickering. If you reread my post, you may find that it has not a thing to do with either your post or with you. It simply says something different than what your post says.
But since you bring it up- the original poster did not ask "what is primer used for?" or "how is primer used in painting cars?". He asked IF primer could be screwed up when painting a model car by sanding it too much. I answered that. Strangely enough, you didn't.
I'd also like to point out that it simply
does not matter how real cars are painted. Even if we're using real car paint, we're not painting 1:1 cars. We're not actually painting cars at all. We're painting
plastic toys. This is a bit like using professional grade house paint to paint turtles. It does not matter how houses are painted- if you're going to paint a tortoise (even if you want to make him look like a house), you are going to use a different technique to paint him than you would use to paint an actual house.
I occasionally sand primer with ridiculous grades of abrasive- like 3200 micromesh. When I have, the results have turned out very well- well enough that it justified the effort to do that level of preparation to the primer. There are even applications in modeling where it would be highly advisable. For instance, if you wanted to build a chrome Ferrari-
-I would recommend that you aggressively polish that primer until you can (nearly) see yourself in it. Spray it with a couple of very light and heavily reduced coats of gloss black, and then lightly spray on Alclad chrome. If you don't polish the primer, it does not matter how much you polish the clearcoat over it- the texture in the primer will forever be clearly visible. In scale modeling if you want a perfect paint job, it helps tremendously to make it perfect from the base up.