With the exception of Magic Markers all finishing primers and Paints will add some mass to your models. The end result (and amount added) will depend directly on how good a finish you want on your models.
Turely it is a craftsmanship thing! Early in ones Model Rocketry it's all about getting the birds in the air. Later as you mature in the hobby, model finishing seems to become more of an issue.
If performance only is the key to your flying then one should refrain from spanding a lot of time on finishing. but more on polishing to smooth aerodynamic areas (fins, transitions and Nosecones) removing as much mass as possible while filling draggy wood grain. Lots of methods to attain these results but even these add a small amount of mass to the overall model.
Everything in Rocketry is a trade off; Performance wise better aero often means balancing mass added & removed with Smooth sliding through the air with slick fins and wood parts.
Great looking Static models finishes are great for close up inspection on the field or display but will ALWAYS Cost altitude.
Many like myself Never even think about Altitude when building Sport flying models. We accept the fact that a babies butt smooth multi color model will be heavier then our Naked or Nearly Naked Competition Altitude models.
It's all about what YOU want with YOUR models. Type of paint; be it water based Airbrushed acrylic, Rattle can Enamels, to Super Automotive SprayGun applied Polyurethanes Add considerable mass to the models regardless of application technique. The larger the model the larger the mass increase. Only way to counter this is to remove Mass in other areas, ie thinner fins, hollow wooden parts. less plastic etc.
Primer is very heavy but we are supposed to be sanding off the majority of the primer anyway so it is not as big a deal as one might think.
Most models should have at least a two coat color coat just so it holds up under normal handling, flying and Landings.
Decide what you want your models to look like, Great close up or Good at 15-30feet. Even the worst open grain wood and unfinished bodytubes look just fine from 15-30feet away