How can I conceal deep scratches in my chrome shower arm and make it look like new?
Replace the shower arm. There is no good way of concealing such scratches.
wrap it entirely with something pretty. the something needs to be impervious to water and damp.
Sand the arm lightly with 600 grit sandpaper. Wipe down with alcohol. Paint whole arm with a rattle can gloss silver or gloss chrome paint. Tape some newspaper on the tiles/wall, shower head and ceiling to avoid overspray. Open the doors and windows while spraying. Wear a mask. Good luck.