I scrape it until I can't scrape it any more. Then I scrape a bit more. Thennnn I smoke all that I scraped, and scrape it again, then smoke that again. Usually I'm royally fucked up when I decide to scrape my pipe for resin so I don't mind the tedious monotonousness of the task. I scrape my pipes damn good. Then, I set them in a microwavable bowl of water and boil em in the microwave for 3 minutes or so and some little black flecks rise to the top. Then, my pipes look brand new. Dont do this to bong bowls though, as it really weakens the already thin glass. Instead, I recommend just scraping those.