Ah, the joy of bondo!!! You need to use a long "sanding block" about 2 1/4" x 17" long. and start with 60 grit paper. When you sand, take long strokes, that start on the "unbondo'd" surface of the door, and continue all the way across your 'bondo patch" and onto the unbond'd part of your door again. Hold the sanding block at an angle to the direction of your stroke. Give it 25 strokes, then change the angle of the board to the opposite of what you were holding and then take 25 more strokes. Then change the direction of your strokes by 90 degrees, so that you are sanding "crossways" to what you were doing before, and repeat the above. Then switch to 120 grit paper and repeat. Then switch to 180 grit and repeat above. You are attempting to 'feather edge" the patch of bondo so that there is no transition that you can feel with your fingers between the bondo patch and the surrounding steel. If you can not feel the transition, then you should not be able to see it after a coat of primer.