Water damage is a BIG problem. Even if it was distilled water, if it was powered on while wet, lots of thing can be fried making it an impossible economical fix. If it is salt water, coke, or other water with salt or sugar, it must be completely washed with fresh water and fully heat dried. That stuff will eat thru wires.
A POST fail should give you some clue as to where the problem is with beeps or flashes. If not, that is a hunt and peck fix. BUT, I don't know what you mean by "POPS".
The volume slider is is obvious.
RUST is oxidized iron. What you are likely seeing is dried contamination, unless it is on a structural steel item. If it is structural, there isn't a need to remove it. If it is contamination, the process ov removing it must also neutralize the source. If you can scrap it off, then wash it with clean water and fully dry it b4 powering. You might also put some vinegar on it prior to washing. That will neutralize any alkaline substance - expecially good for battery corrosion.