I have had success sanding rotors to resurface, it helps get all of the bedded pad material off. I think this is important if you’re switching pad compounds or you’ve been dealing with contamination.
Some tips: Wear latex / rubber gloves to keep all oils / contamination off the rotors. Clean very thoroughly with acetone first, alcohol second to remove all residue possible. Brake Cleaner works too but I always follow up with Alcohol. Use clean new wet/dry sandpaper, I think I used 400 grit but possible more coarse (I’ve heard of steel wool too but haven’t tried it). Easier with the rotor off the bike and a small sanding block, I went in two directions trying to create a cross hatch pattern. But, have to admit I’ve just done it by hand on the bike too. When you’re done, clean with acetone and rubbing alcohol again. If there’s any question whether your pads have been contaminated by any sort of oil / lube - pitch them and use new, they cannot be salvaged if contaminated. Otherwise, you can sand down to create a fresh surface, blast them clean with brake cleaner and reinstall. Then, make sure to properly bed in your new pads.
With that all said, I use metallic pads and they can squeak and squeal even when perfect. Some pad compounds will do that depending on temperature, wet or dry, etc.
Edit: Similar to the process I’ve followed