If you look at the instructions on the Brightside paint
you will see that it does you do not Pre-Kote under it when covering over fiberglass. You will only need to prime spots where you have used a filler.
So save yourself a lot of money and labor time and only prime the places you might have used filler on the surface.
As to rolling and brushing it onto the surface. The first thing you need to know is do not buy the one part version of Brightside, it works for spraying but does not work for rolling and tipping. You want to get the two part component version of Brightside for rolling and tipping. I learned that information from the marine painters, I live next to a boat yard. You can use a foam roller but there is more to it than that. You will find good videos on youtube showing a two person team at work doing this paint
method. They are not using the same paint
but the method is the same. Pay attention to what the guy doing the roller is doing to get the the paint more level for the guy doing the brush tipping. Remember to set up a small practice piece on a scrap of painted plywood to refine your team method before you start working on the trailer. Practice before you dive into the work, it helps prevent mistakes.