Using a DA will only knock the bondo down a little faster. You need a good hard sanding block to finish the bondo. After using the sanding block, if you don't have actual guide coat, spray a little black spray paint on it and then sand with the block again. After you sand and if you still see the spary paint, those will be your low spots. Fill in the low spots with a little more bondo. Then repeaqt the process untill the paint comes off evenly. If you don't have a lot of experience, feeling by hand will be hard because you don't know what your actually looking for. Finish the final sand with 360 grit paper and then prime. Wet sand the primer with 600 grit using a SOFT sanding block. For best results, after sanding the primer, prime and block again. Just take your time and you'll be impressed with what a little patience can do. GL