Wouldn't use emery paper on Ali. I use a copper scraper made from a piece of flattened and sharpened 15mm pipe. And a solvent like carb cleaner but thinners work as well.
And polish with autosol.
I use a soda blaster at work, brings whatever you blast looking like new again without damaging the face. Use it to clean heads pistons anything really..
Might be abit expensive if you only want to clean up a few items there about £200 for a kit and then you still need a compressor..
I have a sand blasting cabinet you'd need some super fine sand though so you don't damage surface :-/ never done internal engine parts before in it! Infact... I've not done a lot in it! It's been a storage shelf for the last cuppa years and I never player around with my van, just serviced n mot'd it!
the best method fer piston tops is a bit of petrol on a tooth brush scrub the hell otu of it leave it fer 5 mins then a blarrt with the paraffin gun they come up like new and not a scratch on the surface