Although I've heard about 'sealing' I have yet to do so, though it sounds like a good idea to keep it from yellowing. As to how do I remove all that black, I use my best friend, Steel Wool. It's perfect, and although it can be kind of messy, it works in no time, and if you use it lightly it gives you a nice greyish area- and rough enough you've got pure, smooth white. Plus it's pretty cheap which is always a plus.
Oh yes- this took me, hmm- about a week, most likely I worked on it a little every night during the week then did most of it over the weekend.