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Well its common to spray a over reduced flat black over the body and block sand. This shows your low spots, fill minor lows with icing and slight nicks with glaze. Make sure you are using a low shrink filler. Do not attempt to use any bondo product if you want a quality job. I use USG lite as my main and USG icing as a mid with USG glaze as a final.
I'm late to thread but sanding with a grill cleaning block will make all the hills and valleys stick out like a sore thumb thus making the filling easier