If your hulls are merely damaged by the sun, you shouldn't require any epoxy or glassing products. If however, there's been some structural damage, then West Systems and System 3 epoxy products are hard to beat. A lot of folks use the glass tape to rebuild worn keels with good results.

To liven up all but the worst gel coat oxidation problems(bleached hulls), sandpaper works very well. Try 600 wet or dry to remove the chalky top layer. Colored hulls make it very easy to tell when you've gone far enough. Once you've removed the oxidized layer follow up with 1000 then 1500 wet or dry to pre-polish the gel coat. Finish the process by using 3M rubbing compound designed for 1500 grit abrasions. Experiment with an orbital polisher. I like to start an area very wet and finish with the pad very dry. I've used this same process with painted surfaces with good results.