Even new gel coat, which is just pigmented polyester resin, is relatively porous, plus the resin itself isn't really water proof, and it only gets worse with time and UV exposure if not protected. If the boat is in North Dakota, the very labor intensive scrub-wet sand-compound-wax-buff process described in that great link DonV posted might work, but if you were in S. Fl., you'd get to do it all over again in about 6 months!
Although I was admittedly skeptical when I first saw a boat-show demo of it, I've been using Poly-glo for about 7 years now, and I'm a believer! Some minor touch up every 1-2 years sure beats the hell out of waxing every 3 months that it'd take to keep it looking as good as the Poly-glo! They also make some aerosol stuff called Poly-strip that does a real good job if you need to remove it. The cleaner used with the 3M pad in the kit will do a pretty good job of removing heavy oxidation, especially if used full strength, but it sounds like some light wet sanding of the topsides might be advisable, depending on what it looks like after the initial cleaning. Start with the topsides and work down, and use some good nitrile gloves! That cleaner is strong stuff and latex gloves don't hold up very well to it! After cleaning the gel coat should look uniformly dull and faded, but the coating, which appears to be some sort of urethane emulsion, will add lots of gloss and totally seal up the porosity. If the coating lasts a year in S. Fl., it should easily last 2-3 years in North Dakota!
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