1. The first and biggest misconception of all. Basing with white will make your color brighter. False. If your choice of primer color is changing your topcoat color, you are not using enough paint. Paint is not transparent, except for candy colors. A fully covered paint job does not rely on the undercoating for its color. If you have any doubt, try a sprayout. Paint half of a part black, paint the other half white, then paint it the color of your choice. I dare you to see a color difference from the white to the black sides. If you see it, you’re kidding yourself. I have proven this numerous times to doubters, under the brightest lights at a car show. the only exception to this is DuPont Chroma Base. That paint is the worst for coverage. The funny thing is, DuPont Lucite used to be the best for overall coverage (but the worst for color match).
2. Primer. People use far too much primer. If your paint is not damaged in any way (cracking, peeling, etc.), there is absolutely no need for primer or primer/sealer. If you strip the car to metal, you of course need primer. If you do any repairs, you need primer. If you are doing a scuff and shoot, leave the primer in the can. Adding primer to an already painted vehicle is just adding more layers of possible future problems.
3. Sanding color off. I once had a conversation at a car show with a guy who swore he did 100 coats of color, but he sanded between every coat. I explained to him that he basically had 2 coats of paint on as he sanded off the other 98 coats. There is absolutely NO reason to sand between every coat of color. Even back when I used to spray lacquer, this seemed like a bad idea. It was and still is. It is a complete waste of time and money. Here’s a tip. Urethane colors need no more than 3 nice wet coats (I use PPG, others may vary). The only reason to sand color is if there are problems. If you did your prep work properly, there should never be a reason to wet sand your color coats, ever. If you spot a problem while spraying your first coat, you have two choices. 1. Stop immediately, fix the issue, then continue, or 2. Complete your 3 coats, then fix the issue and blow in the repair, then add one final coat to make everything nice and even, then add your clearcoats. This is how I do it.
4. Wax. Never, ever wax a freshly painted vehicle. Wax seals the pores, which is good for cured paint, but very, very bad for uncured paint. Paint releases solvents as it cures, requiring pores be left open. Use glaze for the first 30 days or so. After 30 days, you can wax it every day if you like.
I am sure there are more. If I think of them, I will add them.