I once did a Glas Ply 17 - the white upper portion above the rub rail. I scuffed it beforehand with sandpaper and used an airless sprayer with the appropriate tip orifice, having already thinned the gel a bit for good application. All I can say is if you do it yourself, absolutely add the correct amount of a proper surfacing agent, and be prepared for more sanding, rubbing, polishing than you have ever done in your life...