To do a drop coat you turn your gun pressure down about 10 psi, pull the gun back from the panel about 15 inches and spray a fast fog coat over the panel to even out the metallic. It also is effective at removing striping.
THE CRITICAL PART TO A DROPCOAT is you must do it IMMEDIATELY after spraying the basecoat on a panel. The basecoat must be wet. By immediately I mean as soon as you finish spraying that panel. Drop the PSI, pull the gun back, and sweep over the entire panel real quick. It is essential the panel is still wet so the drop coat melts into it. If the base has flashed off then the drop coat will not melt in and will cause adhesion issues for the clear.
I always use a drop coat because it saved my butt the very first time I ever sprayed a blue metallic. That was thanks to Barry who told me to do it. I’ve never tried not doing one since then, but it’s so easy to do, I do it just to ensure an even final coat.
Unless you have achieved God status—which is not me—a dropcoat might still be a good idea for a novice with metallic.
Let me put it this way—I can tell which cars at shows were (probably) painted by the owner, and they would have definitely benefited from a dropcoat.