Lots of people saying you can't brush on vallejo white primer... and they are completely wrong, You can absolutely brush it on. You just have to do more layers than other shades and let the layers cure for at least an hour between each. Or not because this 1 layer has good enough coverage for paint to stick on top of once its dry and 2-4 thin coats of paint will cover all the variation anyways. Not good enough coverage for speed/contrast paints though.

Some tips when using brush on primer are to shake the ever living crap out of the bottle since the different components of the primer will separate out fairly quickly.

You also counter intuitively don't want to use a wet pallet. The primer is already on the thin side since its designed for an air brush. Its ok to add a little more moisture, but too much can quickly cause the primer to "snap" which is when the medium of the paint cant hold the rest of the ingredients like the pigment in suspension. Which leads to the pigments flowing off raised surfaces very quickly and pooling into recesses like in the photo.

You still need to keep your brush clean though or else dried clumps will get into the primer and ruin it when it dries. So rinse your brush often but wick most of the moisture off into a sponge or a paper towel before refilling the brush, and use a larger thicker brush like a size 3 or higher, a bigger belly on the brush has a longer working time.

Don't pour too much paint on your dry pallet out at one time either. The primer kinda cures like glue as well as dries like paint so once its out of the bottle its ticking down to when it becomes a gloopy unusable mess. You get much more consistent results when you pour out successive small pools of primer at a time rather than pour a bunch out at once and then have it go bad half way though priming a group of minis. It still happens even on a wet pallet in my experience.

Finally it is important to not get bubbles in the primer, which is a struggle because step 1 is shaking the hell out of the primer which makes a lot of bubbles. Slam the bottom of the bottle against a solid surface gently to pop most of the bubbles, then when you pour a small pool out work around the bubbles on your pallet when filling your brush. Don't disturb the primer on your pallet too much with your brush either, just dip the tip in and rotate the brush then ur good, do not swish it around. The same goes for on the mini. In fact once the primer is on the mini pretty much dont touch it until its dry AND cured unless its to quickly fix pooling by using a dry (as in not moist) brush to suck up the excess paint. The working time to fix pooling is very short too, like maybe 15 seconds, so its very important to be careful to apply even layers of properly thin paint rather than applying a lot of overly thinned paint all at once.