Polyester primers are really for where high-build and filling properties are the main requirements - "over fiberglass, metal, plastic and wood [and] can be topcoated with all types of finishes: acrylics, lacquers, synthetic enamels and two-component urethane coatings" - ie: just about everything except gelcoat!
(See, for example, the product at: http://www.tcpglobal.com/kustomshop/ksppolyprime.aspx
About the most widespread use I would guess would be in plug-making: the 'boat' shape (etc) made from light, stable timbers and from which a mold is later produced.
But even in plug-making the polyester filler coat, having been sanded to about a 400 grit finish, would then be finished with many coats of polyurethane.
Though there is no reason why it would not work! The point is however, that gelcoat is for applying to the aforementioned mold, once it is obtained from the plug and being the exterior surface of the hull, and indeed, the mold itself when 'laid-up' on the plug!
In short, therefore, gelcoat is not really a paint, it is a thixotropic resin: specifically, the external resin of a GRP structure such as a boat hull or its mold. It might sometimes be retro-applied say, to fairly extensive abrasion damage to a vessel's topsides in localized areas for example. And where it is applied to the surface of a mold by spray as the first process in 'laying up' a new hull (etc), specialist equipment is used which is designed for the purpose.
And it is not a good idea to add solvents (acetone, for example) to it as quantities of these that have not 'flashed off' prior to cure are ultimately retained in microscopic voids in the gelcoat. Weakening it, reducing its imperviousness and, in vessels intended for longer-term immersion, leading to osmosis (though not a problem with a board of course).
If you want to just restore the existing gelcoat to new condition, then you might like to read my answer at: http://uk.answers.yahoo.com/question/index?qid=20100811053040AAnXfzX
But if you want to give the board a paint job, including using spray, then the answer at: http://uk.answers.yahoo.com/question/index?qid=20110227050102AABJbgk
might help. (If you go for polyurethane, two coats should be sufficient for a board).