Hey Jay! @ssgt
I'm only speculating here regarding whitewashing a model tank. I HAVE whitewashed a real tank and I'd do pretty much the same to a model. I'd use a white water-based paint, thinned so that when applied, it doesn't cover totally but leaves streaks and lightly-coated areas. A real tank has (usually) a really coarse texture paint finish. (The paint itself, not the anti-skid material applied at the factory on the usual areas where the crew is likely to walk.) Our M1 Abrams tanks (and our factory re-furbished M-60s & M48s) are painted with special paints, the newer types being resistant to the effects of chemical warfare. It's known as CARC paint: "Chemical Agent Resistant Coating.") It has perhaps THE most matte finish I've ever seen from any type of paint. Those paints are also very tough. I once had the opportunity to shoot my tank with a handgun of 22 caliber: all it did was leave a slightly blurred streak on the paint finish.
As a side note, whenever we needed to "touch-up" our tanks from the inevitable scratches or chips received during maneuvers and day-to-day work accidents (like dropping heavy steel tools onto the turret or hul during crew maintenance) we used standard VERY special (non) CARC paint - Krylon spray paint in cans from the local base Exchange or civilian hardware store! Ha!
Here's a photo taken of my early M1 with whitewash camo applied.
1 attached image. Click to enlarge.