paint motor with a brush?

Depends on what type of finish you want. I have painted many things with all the pains of spraying - done several with a brush. I have had many many complements from painting my old Ford truck with a brush and none for all the pains I took on other surfaces. Remember - Its the eye appeal and what the part is. A nicely painted valve cover will certainly be noticed - a cast block will not. The rough finish seems to reflect more color from a distance. I have been painting my old 40 Dodge 6 - in car - with a brush and on cast it looks really great. Better than rust. Just depends on what you are painting, the type of product you are using, and the care you take to do it.
Beauty is in the eye of the beholder.
Just FYI.
Later,
Bcudamatt