Scrape as much as you can using SMR's technique. I've made scrapers out of a piece of hardwood (maple or oak) by sanding a chisel blade on one end. Works better than plastic, in my opinion. Then use mineral spirits or isopropyl alcohol on dampened rags to clean up the residue. It may take awhile.
You can use a metal putty knife or scraper if you like, but be VERY cautious about damaging the soft material underneath. It only takes one OOOPS! to make a hole. On the other hand, you are probably going to put lap sealant over it anyway, and that would seal any slits or gouges.