By progressively filing down the highs and tapping gently up the lows to make them highs the panel surface is slowly brought back to its original condition. Care must be taken in not creating a large high spot by chasing up the lows. The end result may have all the appearances of having got the dent out - but you may find that the whole area is now too high. This is where a few heat shrinks may be needed as the metal is too stretched . So work slowly and constantly check that you are not creating a large high spot. Highs can be tapped down again with either the panel hammer or the use of the slapper again. You dont want to concentrate on small areas if this is the case - rather bring the whole area back down.