Interesting, I thought that automotive paint was meant to be sanded in stages ending in super high/fine grit?? For base coats are you just supposed to lay on a ton of coats then sand down to super fine before spraying clear? At what point are you supposed to smooth out those deep 400 grit scratches?
Automotive paint is meant to be sanded but only after application. Base coat should not be sanded and should require only one to two coats. Clear should be applied in multiple coats 2-4 based on your plans to cut and buff.
400 grit scratches will not show through and help the paint adhere to the surface. Once the clear is cured, and in the case of Rustolium paint it will take weeks if not months, you can wet sand. Start with 800-1000 grit and go up from there. Then you can start the buffing process.
Now, with all that said. You can wind up with a much better paint job that will last much longer and look better by using s simple single stage black from any of the mentioned suppliers, Kirker, Omni, or Nason. These will be proper automotive catalyzed paints that will cure enough to cut and buff in hours compared to weeks. Without the additional cost of a clear coat your total material cost should be in the ballpark of what you're planning to spend now.
Remember, painting is 90% preparation and 10% actual painting. Sand the surface properly and use the best quality product you can afford and you won't be sorry. Also, modern automotive paints are a miracle of chemistry. They are designed to be compatible within a paint line and may not work at all when mixing brands, ie: spraying a different brand of clear over a base. If you try that any number of things can happen including the paint balling up and looking like wrinkle paint. That's why everyone is telling you to try it on something other than your car before you use it.
Bottom line... use a real automotive catalyzed single stage and you will be much happier with the outcome. The "P" sheets will tell you what sanding surface it needs, and what pressure to spray, as well as everything else you need to know to successfully spray the paint. If a paint does not have a "P" (Product) sheet, it isn't really "automotive" grade paint. Might be something you would paint your tractor with, but not a car.
The truck below was painted with a catalyzed single stage Omni brand automotive paint with no clear coat. Cost was about $250 total.
The '41 Chevy below was painted with base/clear. Total cost of materials, $1800.
Not that much difference except the cost of materials. Unless you're going to do a "show" paint job you really don't need to use a two stage (BC/CC) paint.
Good luck.
