While yesterdays rocks were an improvement I decided that they were probably too uniform in colour, so I've had yet another go. In fact I had two castings to hand so I've used the same approach to paint two new rocks (at the front) in the same way to check the process is repeatable.
The two new rocks were both painted by first randomly covering areas in stone gray and concrete. Once dry a thin wash of stone gray was applied to the entire rock to both darken the colours and to blend the areas together. This initial base coat was then sealed using scenic cement before a thin black wash was run over the rock with extra black added to highlight obvious cracks and features.
This seems to give a similar overall impression to the rocks I painted yesterday but with a more natural looking variation across the surface. I think they look a lot better and hopefully I won't change my mind again tomorrow.