Is rare and not typical of production type boats. I have encountered it sporadically. In each instance the boats were manufactured by companies who normally produce a very good product. This is the one occurring frequently and which is the one most of us have seen evidence of and which is the topic of this article. Let's examine the procedure.
Water migrates through the gelcoat.
This takes time. The speed depends on the thickness and or the porosity of the gelcoat. In fresh water, the effect can be quicker because fresh water is thinner and lighter than salt water. Now, some manufacturers, such as Hunter Marine, do not allow sanding of the gelcoat. It will void the warranty against blistering of the gelcoat.