Before having the wings and tailplane spray painted, they first need a coat of primer to ensure an even surface and fill the structure of the glass covering and any pinholes in the glass covering. This is a terribly messy and smelly job, best done in a well-ventilated area on a not too cold/not too warm day. Not one of my favourite bits. I apply the primer using a metal “Japan Spachtel” (Japan or Surface spatula) and a paint brush. Note that this primer is extremely agressive and will dissolve plastic containers as well as normal paint rollers. I also use a good mask while applying it.