For the hood louvers I painted them and while they're still wet (just tacking up) I use a rag wrapped around a pipe that has been dampened (not soaked) with acetone and roll in the opposite direction as I wipe the paint off the top of the fins. The reason for rolling the rag as you wipe, is the clean part of the rag will pickup the paint, as it gets too much paint on it, it will only smear the paint and not lift it off.
Another trick is to apply a thin layer of thick grease (Q-tip or tooth pick work) where you do want the paint to go and wipe it off when the paint gets tacky. Do not do this in the sun the grease may run or drip where you don't want it.