8 litres was more than enough to paint my 13 ft Boler
with a roller. I have enough left over for future touchups. I am not sure if spraying will use more or less.
There are gelcoat products available to restore the original finish
but, if you are painting
then sand. sand.sand, fill in holes, chips etc with Bondo and then sand,sand, sand! Use a good quality sandable primer and then sand, sand, sand!
Finally paint, sand lightly, and apply second coat etc.