FWIW, I would suggest studying information about paint brushes on websites that deal mostly with fine artists’ needs. Especially useful are vendor and dealer sites that offer up different brands and types of brushes and which also provide information for comparison between them.
Brushes are designed and made to satisfy a particular need. This largely determines the size of the brush (which has been standardized and is indicated by numbers, the larger the number, the larger the brush and vice-versa), and the shape of the bristles (round, chisel, flat, etc.) and the length of the bristles (which is also related to the medium).
They are also made to work with particular paint mediums (watercolors, acrylics, oils, etc.), which also largely determines the materials that the bristles are made from (natural hairs and their different types, or artificial or synthetic materials). Some brush makers claim that their synthetic bristles are just as good as natural hair brushes for the same purposes. This is debatable and users may have other opinions.
Note that the reason why mink, sable, and other similar natural hair bristles are considered superior is that those animal hairs grow in a taper, so brushes can be made that also taper down to very fine points or tips. Unfortunately, these natural bristles are also delicate, wear out and break over time, require careful cleaning and care, and are expensive. Mink and sable are commonly used for oils, watercolors, acrylics and other mediums.
Squirrel, horse, and hog hairs have a constant diameter, and are also used for brushes. Squirrel and horsehair are generally used for watercolor brushes, and stiff hog bristle brushes are used for stenciling and stippling. They are generally less expensive than the mink or sable brushes. However, some finely made watercolor brushes are just as expensive as their sable counterparts.
The brush shape and quality of the tip are generally the most important considerations for scale model and figure painting. Size is important, but supper tiny brushes only hold super tiny amounts of paint, so the size of the area to be painted is what should be used to determine the size of the brush to be used. The rule of thumb is to use the largest brush that you can and still get the job done. Using a brush that’s too small only leads to excess brush marks and difficulty keeping a wet edge on the work area.
Understand that the paint should flow out of the tip of the bristles like ink from the tip of a fountain pen. So, you must learn to control the viscosity of the paint on the pallet and to recognize when the brush needs to be cleaned to allow the paint held in the body of the bristles to flow out of the tip. If you need to use force (pressing the brush harder and harder onto the surface) to make the paint flow off of the brush, then the tips of the bristles are almost certainly clumped together and in need of cleaning. If you notice a tiny ball of paint forming on the very tips of the bristles, stop and give the brush a “swish” though the thinner and a wipe across a towel.
“Sets” of brushes tend to be less expensive than buying brushes individually, but the downside is that the quality is often only marginal to poor. With paint brushes, you will pay for quality. Although sometimes you will not get what you paid for, you will always pay for what you do get.
In my experience, the best places to buy really good paint brushes are also the same places that supply brushes to fine artists. The brushes that you can get through hobby vendors tend to be of marginal quality. Even the more expensive brushes are best purchased when you can hold the brush in your hand to inspect the shape and quality of the tip and bristles. Buying brushes on line, unless you’re going with a very reputable dealer, is a chancy proposition. The same brush you bought last time and found to be acceptable quality may be damaged or poorly packaged when you get it this time.
Having said that, until you have some hand painting skill and experience, “hobby quality” brushes will probably do you well. Once you develop your skills and experience to the point where you know you cannot advance the quality of your work BECAUSE the quality of the brush is simply not there, then you will know enough to also judge the quality of a new brush before you buy it.
Quality brushes will last years and years IF they are not abused and are properly cleaned and maintained. I have a sable liner brush that I use all the time that’s nearly 30 years old. (I admit that it is an exception to the norm…) All brushes do eventually wear out, so they must be considered consumables. Expect to have to replace them, even the best and most expensive ones, by and by.
Develop some good painting habits:
NEVER EVER use your really good brushes for anything other than regular and detail painting. NEVER use a “good” brush for dry-brushing or any other abusive or rough purpose. Once you damage a good brush, it’s pretty much trash. You can occasionally trim away a bent or broken bristle or two, but usually, once it’s damaged, it’s damaged.
Always load the bristles with thinner before you start a session. Dip the brush in the thinners then wipe it on a towel to soak out the excess. You want to prefill the ferrule and base of the bristles with thinner and not paint. This will help keep the paint out of the ferrule and make cleaning easier.
Always wipe the brush with the bristles and not against them.
Understand that the tip of the brush is where the work is done. Don’t squash or use a lot of force on the sides of the bristles. Generally select a larger brush if you find that you’re excessively using the sides of the bristles to brush out the paint. If you have to lay the brush down on the surface all the way to the ferrule to move the paint, then you need a bigger brush or a cleaner brush or a thinner paint.
Find and use quality brush cleaning mediums. Thinners are OK for use during a painting session, but once you done, you need to use a purpose brush cleaner before you store the brush away. Natural bristle brushes also benefit from using a conditioner on them after they have been cleaned. This helps to keep the hairs soft and flexible (and less prone to breakage) and also helps to keep the tip shape. Leave the conditioner in the brush until you use it the next time. (Starting off by dipping the brush into thinners before painting will remove the conditioner.)
Bottom line: Giving a brush just a few swishes in thinners to clean it is the slow road to ruining it. NEVER scrub or stab the bristles on the bottom of the thinner container to clean them.
If you cannot find a purpose brush conditioner, you can use hair washing conditioner as a substitute.
As you use your purpose brush cleaner (I use Windor & Newton cleaner), dip the brush into it and then set the brush aside for a few moments to allow the cleaner to soften the paint residue. The wipe the brush on toweling by pressing the bristles on their side with the ferrule making contact with the toweling. Rotate or roll the brush side to side as you slowly draw it back, keeping light pressure on the sides of the bristles to squeeze the paint residue out of the point where the bristles enter the ferrule. Repeat this process until there is no color or paint residue left on the toweling. Sometimes you need to allow the cleaner to soak a little longer. If necessary, with the brush loaded with cleaner, you can pinch the bristles between your fingers and roll it back and forth between them to agitate the bristles against each other to help remove the residue from them.
Once the brush is clean, dip it into the conditioner (I use The Masters brush cleaner and conditioner which has to be wetted with water and worked up to a foam). You may find that the conditioner also loosens up additional paint residue, if so, repeat the rolling-pulling cleaning technique on toweling. However, this time, you want to draw the brush back while rotating it to form / reform the tip to a sharp point. Once you have the point shaped, store the brush away.
ALWAYS store your brushes with their bristles up, handles down. Keep the small shipping tubes that usually come on the ends protecting the bristles when the brush was new. These can be reused if you want or need to store the brushes laying down (or you want to carry or transport your brushes to use somewhere else, like on holiday, or to a model club meeting, etc.).
Finally, don’t become emotionally attached to your brushes. They do wear out, and when the tips or bristles get “bunged out,” splayed, or broken, switch that brush to other, more abusive duty like dry-brushing or terrain and diorama work and break out a newer one for regular and detail work.