if you have a vice this will be a lot easier.
put the tip in the vice. the end of the tip is rolled a bit so once you tighten it in the vice and start pulling, you won't slide the tip out of the vice, it will catch. Once you do this, take a propane torch or whatever torch you have an heat it up. Once you do this, give it a yank [like we all know you do
] and it should come off fairly easy.
Edit: and if you are using 400, i would just lightly scratch it, don't apply much pressure. or use a higher grit.