Is there a primer for brush painting?

Personally I wouldn't consider a primer to be necessary for brush painting.

Tamiya's Liquid Surface Primer is similar in most respects to the finer grades of Gunze's "Mr Surfacer". These are lacquer based products which can be used to fill small imperfections and scratches prior to painting. You can read more about MrSurfacer here, http://www.swannysmodels.com/Surfacer.html courtesy of Matt Swan.

These products can be brushed on and will level out quite nicely as they dry. The significant difference between the two brands is that Tamiya's LSP is completely soluble in alcohol. This means you can paint it on and wash your brush out with metho (which you can't do with Mr Surfacer) It also means that you can fill a seam with it and then when dry, use a paper towel moistened with metho to wipe off the excess - perfect seams with no sanding.

Ok. Cheers for that.

Do you know what thinner is best for Humbrol enamel? X-20 dries it out. 

How refreshing to be discussing something like this!  I've been building ship models for a little over fifty years, and I do the vast majority of my painting with a brush.  (I have an airbrush, but I don't use it often.) 

I'm aware of two good reasons to use a primer - sometimes - for brush painting.  One - depending on the texture of the plastic and the color of the paint, it's sometimes easier to get a nice, even, brushstroke-free finish starting with a flat-finished surface rather than glossy, bare styrene.  Two - depending on the color of the plastic and the color of the paint, it's often easier to get a good, even finish over some neutral color.  (Covering black plastic with a brushed coat of bright yellow, for instance, can be quite a challenge.  It's likely to take several coats, which can start to obscure detail if you're not extra-careful.)

I keep a spray can of Testor's flat light grey enamel near the workbench.  If I run into a situation where I figure a primer coat can help, I give the parts in question a shot of that stuff. Brush painting light colors over a nice, smooth primer coat can indeed make all the difference.

It should not be necessary to use a primer just to make the paint stick to styrene.  Modern hobby paints are specifically formulated for that purpose.  If your paint isn't sticking, something's wrong.  Either there's mold release on the plastic (if so, wash the parts with dish detergent), or the consistency of the paint isn't right, or you've got a defective tin of paint.  (It happens.)

My personal preference is for acrylic paints (my favorite being PolyScale).  But I think there's a pretty general consensus that plain old "paint thinner" from the hardware or paint store makes as good a thinner for enamel as anything else - and costs considerably less than any thinner sold in a hobby shop.  If the smell is too much for you (as it sometimes is for me), you might check out an art supply store.  Companies like Windsor and Newton and Grumbacher make a variety of solvents for oil paints; some of those solvents are nearly odorless.  (I bought some allegedly "odor-free" paint thinner at Lowe's once, but my nose couldn't tell the difference between it and the ordinary, stinky stuff.)

Good luck.