Just an idea, I tend to always use an old can of car spray, put a blob of blue tack on the top and press the models roof interior onto it, you can blue tack things like bonnets etc to the side of the can, this ensures all parts to be pained get done at the same time so you dont get variations in shade. Using this method sometimes gives the appearance that you have a circular shade difference over where the blue tack is but this usually dissapears when fully dry, not always but 99% of the time, depends what paint your using.
