It really depends on the primer, and size of the jobs. I paint cars for a living and use a 3m spray gun for primer and sealers that has disposable air cap and fluid tip sets. I generally am only priming small repair spots and I use 1.2 tip for primer and sealer. The smaller the tip the more atomization you get. It will lay primer and sealer very smooth but they tend to be a bit slower than a larger tip size. One bad thing about larger tips is you generally need to increase pressure to help atomize heavy primers... and you do that two ways (reducer,and air pressure). Reducing primer somewhat defeats the purpose of primer.... but you can add more coats... and cranking air pressure reduces your transfer efficiency. Usually over the rated psi for the gun and your transfer efficiency goes below 60%.
