Primer what to use

First, the disclaimer...............I am no expert, but I have researched the issue and am following the results of this investigation on my own project.

I think the product illustrated in your link is actually a primer/surfacer. This is used on top of a good primer base because of its high-build properties to fill in minor imperfections in the substrate. I think what you will want to use is a good, quality, 2K epoxy primer as the first coat that goes over your raw surface. These products are formulated so that you can apply your fillers over them without future problems. Unlike most of the spray bomb primer products, the epoxy primer will also protect the surface from the elements if you are forced to leave your project outside while you are working on it.