UV combined with oxygen breaks down the gel coat at the molecular level and once the original outer layer is gone, the gelcoat is very porous ... breaking down much easier.

"Vertglas" - buy it at West Marine (the only place I've found it online). You clean the hull, rub it down with something to remove any wax, make one quick pass with a rubbing compound (without wax) and polisher to remove the loose stuff, and then put on 6 quick coats of Vertglas with a foam/"shammy" brush. It goes on like water and dries to the touch very quickly. You could do an entire catamaran easily in an afternoon.

This stuff is a "co-polymer" (I know that's terribly generic but it's all I know) coating. It's not as hard as paint but it's much harder and durable than wax. It does wear away over time and they recommend washing the boat and applying another two coats once per year.

I refinished my 20 year old 24' monohull with this stuff last year and had several people ask me how I could afford a new boat. A year later, the sides still look terrific and the additional two coats made it shine more than I remember from last year. The topsides, however, where they get direct sun, seem to need more than the recommended two coats to get back to where they should be.