To describe something that no matter how much effort you put in to clean it... It will always be sh*t.
Boss: Deep clean the bleachers with soap and water.
Worker: Man, that's like 'polishing a turd'- regardless how clean we get them.. They will always get dirty.
Despite your best efforts, there are some things you just can't fix/improve.
To point out futile attempts to conceal/change the baisc nature of something.
1) Joe: Danny's back in town. He lost a bunch of weight, got a nose job, got hair plugs and now wears really flashy clothes but he's still one ugly motherfucker.