I used to paint them when the engine was assembled, so that the paint wouldn't end up where it shouldn't be however, not all outside parts of it can be painted, ie, the inner lower part near the half moon seal and up near the Cam Bearing, so I was never too happy with them.
A few years ago, I started having them Electroplated in a 'yellow' zinc finish. They come up looking good and stay that way while protected from rusting.
One I did late last year, here the Front Engine Plate and the Timing cover have been done, along with many other engine fittings