As Terry reported years ago.......Rustoleum gloss back applied with a flat blade screwdriver works best. Any pin holes or thin spots can be fixed after it has cured overnight. I found with a brush the brush strokes can be very evident as thin spots.
I know that troweling it on seems uncouth but it works best, just think on yourself as a high end portrait painter......just don't mess with your ear you will need it for listening.
If not black enough, pour off some unshaken top layer from the can, then shake well to raise the black pigment ratio.
The "blackness" has varied a bit over different production runs and age fading of the silkscreen applied paint/ink.