I've used sandpaper in a circular motion. Let me rephrase that - always going around the cover and not across the cover. That's because on all of the ones I've looked at there is a circular texture to them and I was trying to replicate that. That gives a satin sheen that seems to approximate the original, and you can go finer and finer on the abrasive to get the sheen you want.
And I've not coated the result with anything, even though I could powder coat it satin clear. I didn't think it was necessary and I ran that cover for a couple of years w/o it seemingly changing.
Obviously you are going to lose the lettering, but I think we have figured out a font that pretty closely mimics whatever they used, so you can recreate the lettering if you want.
I believe that's the exact can I just moved. yes. it goes on white and comes off black. just keep rubbing. on tough areas I use a patch cut from old denim. I can go through some jeans so there is plenty around to cut patches from.
I planned on using a polishing wheel and various compounds. My plan is to get a high luster lid and have the base painted black. I think for mine that would go good with my stock looking but not stock engine build.
Just wish I could find the larger 5.8L HO air cleaner like you have, every one I find that is reasonably priced out is either missing the rubber seal on the lid or the lid is dinged/dented along the perimeter.