I know your in the middle of sanding the bottom, and your probably finding it a very arduous job, especially if your going to take it all the way down to the gel coat. You may want to consider hiring someone to soda blast the bottom, it may be worth your back
This is a good time to look for blisters and repair them and fair the bottom, then apply a barrier coat before new bottom paint.
Its allot of work, but will add value (done properly), peace of mind, and a great sense of accomplishment when finished.
The soda blasting may not take "all" the bottom paint of, so you'll still have to sand a allot, will be much easier. Be sure your wearing the proper safety equiptment...breathing apparatus, eye wear, and overalls. You should also be using a sander with a vacuum attachment to be friendly to the environment and your neighbors.