Re: Gelcoat or paint?
Everything is a compromise, as products are formulated for better properties they tend to become more difficult to use, difficult may not be the correct word though. When you add other things to regain some of the user freindliness, it tends to degrade the product. Most things you add to gel coat to thin it are clear, which means you are reducing the % of pigment in the gel coat mix. Gel coats rely on the pigments for most of their UV resistance, when there is less pigment the base resin is being exposed to more UV rays, this means more color change and loss of gloss sooner. Most clear products (incuding paint) tend to turn yellow on their own, so you are adding something that will turn yellow in the sun. Many of these additives are also less resistant to water, so when immeresed for long peroids of time they may become cloudy or blister sooner than unreduced gel coat. There is no way of telling just how much it will degrade the gel coat, it depends on the exact brand and type of gel coat, the color and how much is added. Sometimes you will only notice it in the lab when accelarated weathering tests are done with each mix side by side, other times you see the results in less than a year, like when a repair is made on a new boat using poor techniques.
Porosity isn't really the problem with water resistance (it can be one though), but can be something you run into with gel coat. Water will permeate almost everything (even non porous gel coat) and is a very good solvent, when water is absorbed by gel coat, it may, or may not create problems and marine gel coats have been rigorously tested for blistering, cracking and weathering. They have not been tested with all of the different concoctions that are used to thin it though, some have fewer detrimental effects than others, but none make it better.
Patchaid type products are formulated with high quality base resins similar to what's used in the gel coat, so they typically don't reduce the weatherability of it as much as other solvents do.
If you add only enough of whatever you are going to use to make the gel coat usable and keep the boat on the trailer and out of the sun when not in use, you may never notice a difference.
