Just make sure your paint is mixed well -- stir it, rather than just shaking the bottle -- and don't be afraid to thin it, if necessary, to get the consistency you can work with. As with all brushing, it's about technique: lay down a smooth coat, and try to avoid going back over an area that's already been painted. If it needs 'double' coverage, a proper second coat later on will be much smoother than trying to cover it all in one thick pasty coat.