^ All of this, it can take a while.
And are you using a sanding barrel attatchment? because that kind of looks like a grinding head which would be much slower in this application. Also, in the future I would suggest skim-coating much thinner layers of bondo and building it up rather than digging the helmet out of a much thicker layer. Most would agree that this is the easier route that allows for more control and preservation of detail.
Hope you find the results you're looking for :thumbsup good luck!