Not sure I would bother to remove all the remaining tape with a belt sander. Maybe take the edge off, but there is potential to do real harm if it gets away from you, even for a split second.
Also, you could lay your decking near the old tape and use it as a bolster to support the deck, adding a little thickness to the sidewall. (that's assuming you're happy that its solid and secure) And make darn sure you wear a good respirator/filter. That stuff is nasty in your lungs.