I think I just learned that when working with plane models in particular, adding a thicker layer of primer across the seam is benefitial, because then you can sand across the seam, and still retain a 100% coverage of primer, to best get an idea if the seam has become fully seamless or not.

Using 2500 grit sanding sponge works nicely for the seam covered with primer I noticed. Nice and smooth on Vallejo Glossy Black primer (dried for a day).