Chalk might be a better solution than paint. You can get powdered chalk, put it in old water bottles (lots of them), fill up your panniers with the chalk-filled water bottles , ride the route and mark the turns. Be sure to mark the turns clearly, indicating before a turn that a turn is coming up. Also mark the straight-aheads if there's a questionable intersection.

The chalk will actually turn to a kind of glue in light rain, but a downpour will wash it away. It's a great solution because it is not permanent and therefore does not (usually) require permission from authorities. You can use different colors for different routes.

Bike Virginia uses small arrow signs that are quite effective. They have many volunteers to man intersections--typically participants who get free or reduced entry fee.