If you spray gelcoat, you or someone will spend a lot of time sanding and polishing. If you brush gelcoat on, you or somebody will spend a lot of time sanding and polishing.
Sand and fair the CB with a compound good for underwater use. Then apply several thin coats of gelcoat with a brush. Use waxes aka laminating gelcoat. Laminating gel coat does not fully cure in the presence of air. Once you have enough gelcoat on the board, spray with PVA or a good spray wax. This will block the air and the gelcoat will finish curing. Once fully cured remove the wax and start sanding. If the surface is fairly smooth you can start with a fine grit 220 or higher, if less even start with a a coarser grade. Keep working your way up to 1000 grit and then use a rubbing or polishing compound for the final gloss.
Alternatively use the same process with an epoxy based paint, like VC Performance. VC Performance will form a very hard polishable surface. Its main drawback is the fairly narrow window to sand and polish the surface. After a couple of days the surface is extremely hard.
You might find this video helpful. Andy has other videos on painting and working with gelcoat too.