In my experience, the white color is often a waxy (solid oil material) build-up, left behind by detailing products.
For wax-like buildup within the crevices and cracks of the grain, I use a pine sol solution (or any household degreaser that rinses clean) and a scrub brush, with plastic bristles (never metal). I may work one section at a time, trying to hit the area with the bristles from various angles, top to bottom, left to right, diagonally both ways, then rinse with the garden hose and a stiff spray. I will even rinse with hot water from my utility sink, if I think it will clean better.
Alternatively, placing a NEW layer of an oily solution, such as armor all and many other detailing products, will also "hide" the white gunk in the crevices, but will reappear in time, as the detailing product dries and ages.