[WDShaffer]

Clay bar is used for cleaning good paint to avoid polishing.
You are trying to remove layers of paint and maybe filler to reveal the original paint. That is gonna be tough. To get filler to stick, a coarse sandpaper or grinding disc should have been used.
Unless the current paint job is a rattle can special and no filler, your not going to have perfect original paint under there. The polished patina look is popular, so a coarse cutting compound and then polish will give an interesting result.
Using any sandpaper over 1,000 grit will let you slowly cut through the paint, but the metal will have highs and lows, so will the paint...leaving anything from raw metal to the current paint job as the final result.