 Now, you can certainly also work with this door upright, but the job is a lot easier and possibly more effective if you take it off the hinges and work on it horizontally, like on sawhorses, for example. Not only will this let you do a better job sanding, it will be easier on your back, and you won't have to worry about the finish dripping. Removing that door might not be as difficult as you fear, because once you pull out the hinge pins, it comes off pretty easily easily.