The goal is to have perfectly flat clear coat, with no texture, no ripples, no imperfections. To get there, the clear coat must be sanded with hard blocks and aggressive enough sand paper, 600 grit. If perfection is the goal, this is true, no matter how many coats of clear are sprayed.
The question then becomes, how much clear is needed to be able to sand it flat. The answer depends slightly on what clear coat it is, but generally, 4-6 coats will be needed.
The next common question? Is all the clear sprayed at once, or is it sanded and cleared again, a ‘flow coat’. The answer is all at once, and here’s why. When wet sanding clear coat, it cannot be sanded past the layers that were just sprayed, even if it’s more clear coat underneath. If you break through into the first round of clear coat, you will see a dull edge from the previously sanded clear coat. It will look like a bad blend edge.
All this means that no matter what, 4-6 coats of clear are needed, sprayed at one time, in order to have enough material to sand it flat enough. So adding a second round of clear coat only adds extra solvent, extra material, extra time, and a little extra die back from all the additional solvent.
The important part is knowing the clear coat you’re using, and being very particular with it’s flash times, and not piling heavy coats on.