I used Dad's paint stripper (suggested by Randy Ryan), after acetone failed to work fast enough. I worked more than an hour to remove silkspan from one (of four) panels of my 500 in2 "Primary Force" wing. The paint stripper did the other three in a fraction of that time, with less damage to the balsa from my prying. I don't know why the acetone was so less effective, but at least for my Brodak butyrate, the stripper was superior.