ORIGINAL: squeakalong
One of the last things I do before covering an airframe is take a Q-tip and soak up a bit of water with it. Apply it to each little ''ding'' and bit of hanger rash and then use a hot trim iron run over the moist area to quickly raise the woodgrain. Do this over small areas at a time. Then go over the entire airframe with 320 grit sandpaper on a foam pad to remove the raised grain prior to covering. Using the Q-tip keeps the water in small concentrated areas and the heat does the rest.
Soft landings,
Joe
As an extention to this technique...
You can inject a small amount of water or ammonia into the dents/dings in your wing tips or leading edges that have already been covered on your old battle horse and steam them out too. If the tip is film covered, you may have to then pop the bubbles and re-iron the film covereing. If painted you can't usually use as much heat so the size/depth of dents/dings to be removed is reduced. painted surfaces need to be perforated with a small needle prior to heating or the steam will lift the paint....don't know how to fix that short of spot repair involving knives and repainting etc.