The outer surface of a fiberglass boat is typically made from a special resin called gelcoat. The gelcoat protects the hull and gives it its color and shine. Typically, exposure to the sun’s UV rays over time erode the gelcoat surface leaving it dull and chalky. Fortunately, the high-gloss can usually be restored. Don Casey, a 30 year veteran in the boating industry sums it up best in the following points:

​Wax
     Keeping gelcoat coated with wax-starting when the boat is new-is the best way to prolong its life. Regularly waxed gelcoat can retain its gloss for 15 years or more. The real purpose of a coat of wax is to protect, but wax also has restorative properties if the gelcoat is not too badly weathered. We recommend Maguires Premium Wax.
 

​Using Rubbing Compound
     If the gelcoat is weathered so badly that polish fails to restore its shine, you will need the stronger abrasives rubbing compound contains. Wax on the surface can cause the compound to cut unevenly, so the first step is to remove all wax by "sweeping" the surface in one direction--not back and forth--with rags saturated with dewax solvent or toluene.

A  rubbing compound is selected that is formulated for fiberglass and is applied by rubbing it with a circular motion until the surface turns glassy. The gelcoat on your boat is about 10 times as thick as the paint on your car, so compound shouldn't cut all the way.

After the surface has been compounded, the boat is then polished, then re-coated with wax to provide a UV protectant to maintain its high gloss.  Providing the gelcoat has an adequate thickness--your boat might have been compounded previously--this process will restore the shine to fiberglass in almost any condition.