I have three prehung doors with built-in aluminum door sills. They are 15-20 years old and starting to look kinda crappy. I've tried cleaning them with household cleaners but it didn't improve the appearance much. One is between my LR and a newly renovated 3-season porch so the bad appearance really contrasts with and detracts from the new floor. Any thoughts on making them look better? I suspect paint would not wear well.
If you're referring to tarnish, try a metal cleaner marketed for aluminum. Most thresholds are raw (not anodized or clearcoated), and you can simply polish away the oxidation with the right product.
You could also brush the surface with a drill or rotary tool and a wire brush to give it a textured appearance and make it less slippery.