When I’m painting an intricate surface with a base coat, I do the first layer with really thin paint so that it will run into the cracks. Once that’s dry, I follow up with thicker paint (which might still be thinned a little, just thicker than the original “infiltration coat”). A lot of my bases have parts with sand or ballast glued down then primed. This is the best place for a very thin initial coat so that you don’t go crazy trying to paint sand with regular paint.