Myself and EBEACH sprayed GelCoat on our 20's. I have done it 3 times in the past once on a 20 ft sailboat and the other time on my fathers 19 Mako. It can be done and really is pretty easy. The prep work is simple, sand and fair hull using poly resin products and 80 grit paper, shoot gel coat, 2-4 coats thinned with styrene and the final coat with styrene and wax additive. Let cure than fair out starting with 120-220 grit and work up 1500-2000 then compound and wax the last part is the bare and is very labor intensive.
I have only done it with white gel coat I think dark colors would be very difficult but EBEACH did a simply amazing job on his 20 Sceptere with fighting lady yellow.
Besides the cost of time but including a new Gel-coat gun from
Fiberglass supply gelcoat, sandpaper, and DYEKEM, it cost under $350.00 to do my 20 inside and out.
By the way DYKEM is a metal marking fluid used when cutting metal. After you have sprayed the gelcoat and it has cured you wipe the DYKEM on the hull and let it dry (dark blue color) due to all of the highs and lows with the orange peel you know you can stop sanding an area when all becomes white (sanding the DYKEM off)