Gelcoat is simply polyester resin with pigment and some flow modifiers added to it - there is nothing magic in it. The magic *is*, however, in getting the right mix of pigments so that it matches the gelcoat on your boat. You can do this (I have), but it is a truly tedious process and, for me anyway, very very challenging. Instead, I have an alternative for you.
I wonder if you knew that Fiberlay will make up a quart (minimum size) of gelcoat to match your sample. They scan the sample using not one, but three different light sources, take the average of those three results, and use that as a starting point for a manual match. You even get a custom label!
|(Pay no attention to the gelcoat smeared on the outside of the can, and don't let the can fall out of your car onto the pavement - the lid will probably come off)|
Now, here's the tricky part - how do you get them a sample to scan? If you have something that can be taken off your boat that has representative gelcoat on it (a lazarette hatch for example), then you are in good shape. If not, then I hope you have saved all those plugs you cut out when installing instruments, etc.
|Always save those plugs|
Next, you will have a choice to have the gelcoat mixed up with or without wax.
Wax? Why wax?
You see, oxygen is a chain stopper for the polymerization reaction that turns liquid polyester resin into solid polyester resin. That means that the surface of a gelcoat application will not cure where it is exposed to air. When you are making a boat in a female mold, this is a good thing, insuring that the next layer to be applied will bond chemically with the uncured surface of the gelcoat. When patching this can be handy too, especially since gelcoat shrinks some while curing, and thus will likely require more than one application to a given patch.
I chose to have the wax left out. And I bought a small bottle of PVA (polyvinyl alcohol). This is a water soluble plastic that can be painted over the final layer of gelcoat to exclude air from the surface. A simple water rinse removes it.
I will post some before/after pictures later... after the weather gets nice enough to spend time out on deck in the sun.