While doing some research, I came a cross a few interesting tidbits on gel coat and some of the problems with it:
Ferretti is the only company that I know of that applies its gel coat with a brush. Why? Simply because when gel coat is sprayed on millions of miniscule air bubbles are trapped between the gel coat and the mold, leaving a microscopic pits in the boat’s surface. That pit will quickly fill with dirt. Once the boat is waxed a time or two that dirt cannot be removed and the boat will lose its high-gloss shine. Also, when gel coat is sprayed on more styrene or solvent must be mixed in to make it flow through the spray gun. According to the company styrene is an element that reacts unfavorably to ultraviolet rays, turning the gel coat yellow in time.
After the gel coat, Ferretti uses vinylester resin in the first four layers of its laminate to reduce the chance of blistering and to retard water migration. Also, this resin – which costs twice as much as orthophalic resin – shrinks less when curing and is less likely to display “pattern show-thru.” Ferretti uses numerous layers of glass cloth with Kevlar or a similar aramidic fiber woven in. This ads stiffness and strength and reduces weight. The company claims that their laminates are 25% lighter than conventional all-glass construction. Ferretti uses only PVC foam in its sandwich construction above the waterline in hull sides and decks. No balsa core.
And, some forums have some explanation as to why there are certain problem areas escpecially around curves.
You're correct in thinking its the styrene reacting from UV rays.
This is very common on a radius because these areas can be more difficult to spray and the gel coat will tend to be thicker. Thick gel coat can have a higher level of styrene and/or resin near the mold surface which will yellow sooner than thinner areas. Sometimes sanding the surface will remove the yellowed layer and it won't come back, but other times the layer is deep enough that it yellows again.
The areas with striations are typically from spraying the gel coat at an angle and it being blown across the surface, this can allow the pigments and fillers to separate slightly from the resin and styrene. Plus if there are several pigments used to make the color, the difference in weight between them will make them separate.
While the quality of the gel coat will help prevent these problems, it comes down to how well it was applied.
On some molds the design can make it almost impossible to spray it correctly, so it doesn’t always mean it was the gel coaters fault.
We hear a lot of boat companies talking about how they use a superior gelcoat than others. It would certainly be interesting to see any manufacturers (ahem, West) post some comparisons, or any of the magazines (DIY boat).
Also, certain boats have issues that are inherent with their construction, such as a reverse transom, or some exaggerated curves and arches. these shapes are difficult to mold. A lot of overhangs can make it difficult to mold a shape for example.
Also, yellowing is a problem, but not as much as fading. Fading is a big issue that keeps us in a job. The inevitable 'clouds' that appear when a boat is not maintained are a huge headache for boaters (who don't have our number!).