I think the people you see sealing the bottom sanded or blasted to much gelcoat away.
If that happens then you will need to have re-gelcoated or sealed and painted.
But if you can get it off with out harming the gelcoat then some 800 sand paper will shine you right back up.
A good place for answers to this question is the hull truth
http://thehulltruth.net/category-view.asp Thay had a real good right up about it last year.