With regard to the Preval sprayers, one of my good friends (John Bauldry) uses them almost exclusively and had showed me how to do it. They work amazingly well for small jobs. Have to thin the gel with acetone to get it to spray smoothly. The acetone vaporizes before the gel hits the boat so it doesn't impact application very much.

As to the quality of the result - I did a decent size 4"x6" repair on the side of my boat last summer and I challenge anyone to find it.