Gelcoating a repair area is challenging, but can be accomplished. It requires the proper equipment and materials along with some good techniques in order to achieve a nice finish. Do-overs are possible if the results are not as intended.
Gelcoat can be sprayed with a conventional paint spray gun. I recommend purchasing an inexpensive gravity feed, though suction feed will work as well. The gun must be equipped with a rather large spray tip due to the higher viscosity of the gelcoat.
The surface to be sprayed must be prepared correctly. This includes sanding with 180 to 320 grit sandpaper and removing all contamination. The surface must be uniform and to the intended shape, with any transistions between surfaces smooth and consistant.
Gelcoat in a mixing cup needs to be thinned to lower the viscosity to a sprayable level. This can be accomplished with a repair additive, MEK solvent, clear gelcoat, or acetone. Some of these materials are recommended more than others, and acetone especially is one to try and avoid. Before spraying, the catalyst needs to be added at the recommended level.
Spraying standard gelcoat onto an exterior surface will normally leave the surface tacky and difficult to sand. This can be avoided by adding some liquid paraffin wax (about the same amount as the catalyst) or by immediately following the sprayjob with a layer of water-based Partall Film #10. Basically the top surface of the freshly-sprayed gelcoat needs to cure without being in contact with the air. The paraffin wax rises to the surface to accomplish this, or the Partall Film lays on the surface to accomplish this.
The spraying of the gelcoat needs to be accomplished quickly once the catalyst is added. The gel time is running after the catalyst is added. This includes cleaning out the gun. This will be easily learned if you are spraying along and the gun stops spraying. After opening the lid you see a semi-solid mass. Now you have to hustle to get the gun apart and the gelcoat chunks cleaned out before cure.
Acetone is the best way to clean the gun out well. The needle, seat, nozzle, and cup all need to have all residue cleaned and flushed. Any remaining gelcoat will cure inside the gun and may ruin it for future use.
have a good cure and a wonderful result!