I try to make it so I either don't have to sand or do very little. Both color and clear. For one thing, if your color is metallic it's next to impossible to sand it and not mess up the color. You know, uncovering the small flakes, disrupting the way the flakes layed down. So the best thing to do is to learn to handle both this way.
When I paint car bodies I normally use a bottom feed airbrush with a bottle. That way I have plenty of paint and don't have to stop. I also almost always use lacquer. Either automotive touch up paint or fingernail polish. To a degree it works with enamels too but you generally can't thin enamels as much as lacquers. Tamiya's bottled acrylics will spray pretty much like lacquers too. You can even use lacquer thinner with them.
So, you get your mix right. Whatever you normally use. The milk consistency thing or whatever. Make sure you have enough mixed to do the complete job allowing for extra coats. You lay down your light coats to build up color. Or clear. Once you have complete coverage thin your paint a little more, whether it's clear or color, wait 10 minutes or so and spray a heavier coat. Not running heavy but heavy enough that you can cover with one coat remembering that the paint is thinner now so don't get carried away. If everything is going well and looking smooth wait a few minutes and lay down another coat. If you're seeing orange peel or other texture then immediately lay down another wet coat to see if it levels out. If it doesn't, stop, let it cure/dry completely and sand it out then recoat with your slightly thinner mix. You don't have to go back to your original mix once you've reached this point unless your have to strip it all and start over.
If you're shooting the color coats and it looks good or good enough that you can color sand to smooth it out then stop right here and wait for it to dry, then sand lightly to smooth. If it's metallic and you have to sand you'll have to recoat or it won't look right. You can do that with a fairly thin mix, you just need to cover enough to make it look uniform.
But, if you're doing the clear coats, then every 10 minutes or so go back and shoot another coat. Every two or three coats make your paint(clear) a little thinner. By the time you're done your mix will probably be 2/3 thinner and 1/3 paint. You can get away with that with lacquer. Enamel takes much longer to dry and you can't thin nearly as much as lacquer. Tamiya bottled acrylic isn't that much different from lacquer when thinned with lacquer thinner and Tamiya supports using lacquer thinner with those paints.
The point of all this is to make your top coats self-level so you have to do little to no sanding. If you're going for a show car finish then you're going to have to do a little polishing regardless but once you learn how to do this it doesn't take much to get a glass smooth slick finish. If it's going on the shelf and not the show, you can get the paint smooth enough to not bother with sanding/polishing. It will work.
I'm not an instructor and don't have hands on experiencing with teaching so I can only speak from my experience but, when I was beginning and learning I tended to not build up the clear thick enough. Or color for that matter. The cool thing about lacquer is how thin it is once it's dry and how hard it is. Enamels are much thicker and softer. Tamiya acrylic will be thin like lacquer but softer. You can still do this with enamels too, you just can't thin as much or shoot as heavy a coat or it will get out of control.
You have to learn how much is enough and that only comes with practice. When you have a good solid color, give it another wet coat. With clear, give it 2 or 3. Over thinning lets you shoot fairly heavy wet coats without building up too thick because the extra thinner keeps it from drying too quickly and level. Then the thinner evaporates, the paint cures and it's much thinner than it looked originally. You have to give at least 10 minutes or so between wet coats at this point to let it dry but you don't want to wait too long so the coats will bond or burn together. Again, enamels don't quite work that way. You have to let it dry or it will sag with another wet coat.
This is a real book isn't it? Sorry about that. As an aside, my favorite clear of anything I've ever used is clear finger nail polish with Rimmel 60 Seconds being my favorite. It's pure clear and doesn't yellow over time. A couple of bottles is plenty for a single car. It usually costs less than hobby paints and sprays very well. Also, a gravity feed brush will work just fine. Just make sure you keep your paints ready to use so you can refill the cup at any time and don't start a coat without enough in the cup to finish it. I'm just north of Roanoke btw.
Tony