On the Painting Table .... Grey or Gray?

Warhammer 40,000 – Tanks are big and take a lot of paint for good coverage. This is what my latest commission has taught me. Super Heavy Tanks take about 3X as much paint as a regular one. Sentinels while they appear much smaller have a ton of little nooks a crannies that require paint.

There doesn't appear to be much done in these photos but rest assured it's a night's worth of painting just to get smooth coverage on these tanks.

I've considered using my airbrush on these but as of yet I'm not totally happy with the finish I'm getting. The coverage seems to work fine on building an terrain but I fear that it might be to chalky feeling on these tanks. (Yes I know once you seal them it doesn't matter).