Grease on the paint can cause the paint stripper not to penetrate the paint. You should thoroughly clean and degrease the frame before trying to strip it. I've never used Jasco, I use the "Aircraft Stripper" they sell at Pep Boys, or I use Permetex Gasket Remover, which comes in a spray can. Both have given me good results.
I never touch a good frame with sandpaper, not so much because I worry about damaging the metal, but because it takes a lot of primer to fill in the scratches in the metal. Scratches will often show through the paint if you don't prepare the surface properly (priming and sanding with fine sandpaper).