So not a car but a fiberglass Lund boat, green in color, long story if desired, Looking at a few days of rainy an cool, thought I might spruce up the old beast, oxidized and stained, thinking clay bar and wax, but thought I would ask if anyone has a new great all in one easier than that solution, Thanks in advance.
I have some mildew spots on the 71 Johnson motor for the old crestliner, and black stains on the aluminum Grumman canoe too, Who thought rainy and cool could be so much fun for 2 to 4 week a year boats 
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The clay bar will remove grime, the wax will shine it up - sort of. You likely need a gel-coat polish to remove the oxidation, much like car paint, before a wax job. While you probably could use car products, there may be boat-specific products that are better.
Like these;
https://www.westmarine.com/boat-fiberglass-polish/3m
Notice light, medium and heavy cutting compounds for varying degrees of oxidation. Also note the prices. BOAT is an acronym for Bust Out Another Thousand… 
Gelcoat is a different animal. In addition to Westmarine check out Shurhold.com for videos and products, flitz.com. A different approach poli glow,
Polyglow-int.com.
I have used automotive cleaner-wax but success depends on how badly oxidized the gelcoat.
You might want to search your tv cable box for shipshape, tv program for boat improvement.
If you want to clay the boat or anything else get a Godzilla kit instead. It replaces clay with a plastic holder with a mesh base. Same idea, lubricate and rub over the surface. However, there is no kneading, the handle fits your hand, and if you drop it you can wash it off. It lasts for several cars. Its now available at Walmart.
I’ve used it on three new 2016 cars with less effort than clay. I always use it,( I formerly used clay) on all my new cars and then seal them. Cleaning is usually Lucas Slick Mist.
Godzilla had an interesting youtube video using their products to restore very oxidized red paint.
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I hope you have a motorized polisher. I’ve done this job by hand and it is really exhausting. The hull will get hazy again real fast if you don’t seal it and you’ll be doing it again in short order so word to the wise…
If by stains you mean rust stains from the metal bits, like lift rings, that requires a separate procedure.
I hate to say it, but a marine store, or perhaps a boat forum, might yield far more information than our forum. Boat surfaces are subjected to very different issues than car surfaces.
Ah, victim of autocorrect. I suspect you meant Surf City Clayzilla.
I think you need to get the buffer out and polishing compound first, then wax. Wax won’t do much on an oxidized finish.
Stopped by the hardware store, got some starbrite marine polish, 1$ per oz but looking good, just the part behind the safety strap. Killing it on mildew also 