what is the quickest way to knock bondo down so that it can be finish sanded. I am using 80 grit on a block and getting the job done, but it is a slow process. If I put the 80 grit on an air powered block it works pretty quickly but my compressor runs alot, and I am getting worried about what my electric bill will be. Should I try 36 or 40 grit by hand? What is the proper tool for the job? Thanks.