Try using one of the three inch foam rollers, experiment with thinning the paint. If you google long enough you will find examples of where people have painted entire cars this way.
I needed to get some paint on the top of my '54' Willys wagon a while back and didn't have the time to hassle with spray painting, so I used this method. Came out quite good. Has significant "orange peel" look, but at least it doesn't have brush strokes.

Lee