I have used a heat gun and scraper to get bedliners up alot....but thats also recently done,(within the last 6 years), bedliner and it is still a hassle. I assume that you are either painting or recoating it with bedliner...? If so then just go to town on it, wire wheel, 40 grit sandpaper on a DA works, anything aggressive will take that off and as long as you are priming it you will be ok as a finished product. have fun