Sebby, most of us once painted with paintbrush only.
Well, except for the guys that jumped right in with "all the tools" when they built their first model.
You sure can use a primer or surfacer with a paintbrush, just make sure you have your brush cleaner ready to go when you start. You will also want to get some fine, very fine, and extremely fine sanding films for your primer coat. As you go along, clean your paintbrush fairly frequently, this keeps "stuff" from drying in between the bristles.
Then just follow the tips posted with airbrushers/rattle can sprayers in mind,,,,,in other words, apply primer to check seams and give a good bite for your paint and a uniform color over your plastic, filler, and any metal or resin parts, sand it smooth, knocking off any brush marks, fill again if needed, re-prime, re-sand, then brush your paint on top of that.
You can even get a pretty good gloss coat with a paintbrush, just sand your color coat with extremely fine sanding film, brush on a good "wet coat" of paint and thinner combined, and polish with a cloth, and then cover in a clear glosscoat such as Pledge(Future).
The best way to develop your technique is to use a model car body,,,,,,then do it and redo it until you think you have a presentable car body that you wouldn't mind showing anyone. Once you get that to your own satisfaction, doing a tank or aircraft in flat paints will seem easy in comparison.
Rex