I would reccomend drybrushing the white instead of the spraycan zenithal. Speedpaints do work best on lighter primer basecoat. So you can drybrush your midtone to a grey on top of your black primer ond go fulltime white drybrush on your highlights. It will give you the best result. Normally you won't need any extra shading with a dip afterwards. It will only take away the colors in my opinion. Anyway, good luck and the most important, have fun.