Late, but adding my $0.02 worth. I am a research chemist by trade, and I trained and was certified in all the major lines of automotive refinishing systems a very long, long time ago. For years I have been a devoted wax-head. That changed this year - I am now trying a sealant "polish", specifically Yacht Brite's Pro Polish.
I just finished the spring buff and fluff ritual on our Sea Ray 330 Dancer. Last year it was heavy compound, fine polish (both 3M) and then Meguiar's Flagship Premium wax. The result was tremendous, and the Meguiar's product was easy on, easy off and gave a great look. But, as time passed I found it tenaciously collected dirt, which fed mildew growth in the coating. For the first couple of months a simple rewax of the affected areas did a good job of re-cleaning and re waxing the area. But after that it become increasing difficult to get the dirt, streaks and stains out. Summer weather here is not very conducive to rewaxing, so I let it go.
Last week I found getting all mildew stains out to be quite a challenge - it took a lot of aggressive cleaning. The Meguiar's product is easy to use, gives a great initial result and is extremely durable - I still had a well bonded coating this spring. But the dirt and mildew issue is a real problem.
This year I tried Buff Magic as the oxidation issue was mild. Boy am I impressed - its a great product. I have followed with the Pro Polish and hope this will do better as far as dirt and mildew go.
Added comment about using cleansers on gelcoat finishes. This is basically the same as compounding. The result will depend upon how aggressive the cleanser - "the "soft scrub" versions are generally a finer abrasive than the comparable dry versions. One thing to watch out for though is cleansers with bleach. Bleach is a fairly powerful oxidant and can lead to increasing the rate at which gelcoat oxidizes and can lead to oxidation deeper than just the surface. Two cleansers I have found useful on fiberglass are Bar Keepers Friend (dry but quite mild and bleach-free) and Liquid Zud (soft scrub type), which is bleach-free but contains oxalic acid which is great for removing rust stains, mildew, etc.