Thanks Ernest
The tank is not primed in fact. The gray is the plastic color. I almost never prime any of my models. When I preshade an aircraft, I do it right over what ever the plastic color is. This is nearly always light gray except for the Eduard kits that are kind of khaki. Since the surface is so smooth it is really easy to control how thin the coats of paint go on and there by how much of the preshading you can see. Like I said above… with having to hit the thing from so many angles, I really don’t it can be easily done. I had noticed that an armor WIP is rarely preshaded. I may have just started building but I was always lurking.
I think the next time I am going to down a base coat of the color without lightening for scale. Then go back and randomly fill in areas with the lightened color, leaving a darker edge around panels etc. Almost the same as I did with my a/c builds. Preshade (some of the time), base coat, then a random/splotchy application of lightened color for a faded appearance.
The effect is subtle and less visible in the pic but you can get the idea.
Not too sure about the sand primer though. If the OD is thin enough to see the preshading it might also give you unwanted color shift too.
