Filters can soften these edges. Basically use heavily thinned versions of the base color, they are applied and allowed to dry and repeated until the effect is what you desire.
A filter is just like a glaze, a semi translucent layer of a color. Often times someone may apply a glaze to their painted wall in their house to shift the color and/or give it a deeper, richer surface, or an antique look.
Another way is to lightly mist a thinned version of the base color from the airbrush, which can have the same effect if done carefully. The good points are that it's faster to apply than hand brushing, the only drawback is that it is harder to control than a filter applied by hand brushing.
If you can, practice on something else, or in an area where it wont be seen or covered up with stowage/dirt/etc.
