Skim coat application

I can give you my opinion FWIW. I do all my own body & paint and from what you describe you should not have or want to skim coat the whole car. Opinions vary but after I had mine sanded as far as I was going to, I painted the whole car with 2 part epoxy sealer. Then sand for adhesion and start with a heavy bodied high build primer and start blocking and priming with that using filler or glaze as required. When you get close, use a "guide coat" to locate low spots and prime & block until the car is straight. You will hear lots of ways to do your job but that is how I get a car ready for color. The color application is another conversation.
Best of luck with your project, nothing quite as rewarding as an owner applied nice paint job!
DR:coffee2: