Where the coils sit, exposed to the sides of the engine bay, even though they are high up, still allows the elements and again moisture to attack underneath the coils. No need to grind out the marks deep in the castings, as these will always be covered by the coils themselves. There is a happy medium you have to reach, as you cannot get them perfect as very old now. You would have to buy new rocker covers and polish them for 100% perfection.