If you have a small painting space and/or live in a climate where outdoor priming is often not possible, you can still get great results with brush on primer. But it takes a little know how to make the most of it. Here are a few tips I have learned.
1. Primer is not paint. It can be hard to get even coverage with light color primers. But that is okay, think of primer like the "glue" that sticks the paint to the model. As long as the surface is coated you are okay, even if it looks crummy. You can simply use your base coat paint over the primer to give you the even coverage you need.
2. Paint on Primer can bubble. Lighter colors of primers especially can form little bubbles while brush it on that will dry and leave little "hole" on the model that do not look good. You can avoid this with a few simple methods.
First, don't load up your brush with primer. Use just enough that it goes on easy, but does not run, pool or splatter.
Second, use slow, short and gentle brush strokes. This will minimize air bubbles.
Third, when painting over rough or textured areas air can get into the primer and cause bubbles. Over these areas use a gentle, shallow dabbing motion (not stabbing) this will prevent bubbles.
Fourth, it is really hard to get white and other light primers not to bubble. Do the best you can to minimize it but as you prime keep an eye out for bubbles. When you see them, you can easily get right of them by blowing on the model with quick puff, like blowing out a candle. Sometimes it might take a few puffs but will take care of any bubbles. You can do this as you go or at the end, as long as the primer is still wet. Warning! Be mindful or your workspace. Some primer might come off the model and splatter below. So, if you are working on a surface, you don't want paint on then cover it with something like a newspaper.
Last thing to consider is if you are really not happy with your brush on primer results consider switching to a darker color. Black is very forgiving with bubbles, coverage and evenness. Black primer can be used with any color scheme really, even white and can give some real depth to the model. It is also perfect for the "slap chop" method of painting.
I hope this helps and if you have any suggestion for priming with a brush let me know!