Is the paint
just dull or is it damaged, flaking or scratched?
If it is just dull, you can simply wax it.
A popular solution is using Zep floor wax (que tu trouveras chez Home Depot).
There are a lot of threads on this forum about it, here's the main one:
Check this out NOT Poliglow...Time will tell..
Of course a thorough cleaning/scrubbing is required before waxing.
If the paint
is not in good condition enough for waxing, you may consider stripping the old paint
and either repainting or leaving it unpainted depending on the gelcoat surface you will uncover. In either case, a time consuming/costly job for sure.