John...Another trick that works real well for me is to spray the area with a guide coat, after you skim coat it with filler. We use aerosol can flat black where I work. As you sand the area out with a block, you will instantly see high spots because the guide coat will sand right off, areas that have guide coat remaining are low. Keep working at it , and don't get fed up. it will definately pay off in the long run.
Another thing to remember when you are sanding filler. If you hit a metal spot that is high, tap it lightly. No matter how much you sand, you aren't gonna sand the metal spot down.
Hope this helps. Good luck
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