I am assuming you are asking this because the surface of the fibreglass is matt due to exposure/UV etc?
First be absolutely sure it is Fibreglass, not ABS or other plastic. If you can see the fibre strands on an inside surface then it will be as you said fibreglass.
You need to go to a yacht chandlers and get a bottle of the special abrasive paste sold exclusively for rejuvenating GRP [glass reinforced plastic]. Plus another bottle of a good compatible polish/sealer. 3M being one reputable maker of these products.
Use this in accordance with the instruction on the bottle; basically this is to pick dry day and "polish" away small areas with the paste until you cut through the matted/oxidised surface. Dont leave that surface however nice it comes up but quickly follow with their sealer.
Its going to be a hard slog but as long as there is a half decent "gell-coat" layer to work on you will win.
Then keep the surface well fed with the sealer/polish to stop it going off again.