yeah plenum was deffinetly the hardest. took about 8 hours to get it that good, and i still want it better, but its ok.
not much elbow grease, just very time consuming and while doing the job with buffing wheels on a bench grinder the alloy get very hot so u have to wear gloves also.
it also makes the job alot easier if you sand all the cast marks out and make the surface area smooth as possible.