Adding primer

That's a bunch of red flags.  I wouldn't use any primer that wasn't specifically made for models, such as Testors/Model Master, Tamiya, Mr. Surfacer, etc.

You may be putting it on too dry.  That sometimes causes the rough appearance.  Try a heavier coat.

As a rule of thumb , I always test on the sprue or a part of the underneath or inside that is not going to be seen . best to stand the can in a bowl of Hand hot water for ten minutes , Then spray a light coat on from about 10 inches away . return the can to the water have a cup of coffee and then re spray . I nearly always spray three coats  in the space of an hour . no splatters or grainy effects . best of luck  ian