voids and cracks
Usually, you will find hair line gel coat cracks in curvatures and so forth. It could be that there is a void, too much putty behind the gel coat, or to thin which is the least of the three.Simply removing the gel coat and applying more gel does not take care of the fix in my books. You have to go deeper to see what it is. Generlly, I have found voids where the glass did not get rolled up against the gel coat in manufacturing or there may be too much putty.I usually dig it out and then do the repair and re gel.The reason I like the dremel as it has so many bits and in small places, it is great. I use it with my N Scale model trains. I feel I have better control with this tool but anything can work.As for gel coat which I refer to, it is a paint basically if you want to call it that. It is hard to match white to white but if you had to get some, look for bright white or a duller shade for the older boats.If you are near a dealer, maybe he has some for your use.