I’m just a hobbyist and these are my experiences…
Really good dry brush weathering is a very subtle effect. When overdone it looks, well, overdone and detracts from perceived realism. I apply dry brushing as the last weathering step on a model.
Here are the tools that I use for the dry brushing:
The paint I use is Tamiya XF-53 Neutral Grey. I also use XF-57 Buff: I apply the grey on black/grey/blue base coats while I like the buff on red/brown/green base coats. I used to use enamel paints for dry brushing, but I wanted to eliminate another line of paint and thinner to stock. I liked enamels because they stay wet on the bush longer than acrylics.
To improve this wet characteristic with acrylic Tamiya XF paint, I add their Retarder (Acrylic) #87114. Tamiya recommends that no more than 10% retarder should be added to a given volume. For simplicity, I have dedicated 10ml grey and buff bottles with .7ml retarder added to each. I mark the caps with a large “D” for easy identification.
For the brush, I use Tamiya flat brushes #87046 (3/32″) or #87047 (3/16″) depending on the size of the area to cover. I use a piece of old flannel bedsheet or old cotton t-shirt to rub the excess paint from the brush. Paper towel leaves fine lint that gets transferred to the model, so I avoid this. Another way to soak up excess paint from the brush is to use a piece of cardboard (but, I have not tried this).
Here are some tips:
I use a brass rod stir stick and touch the brush to the thicker paint on the end of this stick. I don’t add too much to the brush as most of it will be wiped away.
I aggressively wipe/rub the loaded brush on the cloth to remove practically all the paint from the bristles. This is the essential key to dry brushing. Rub a lot, and then some more. The effect on the model must be subtle.
After rubbing the brush, I start in an obscure area of the model to see how well I have removed the paint. If it goes on heavy, I rub some more. If it looks nice and light, I continue very lightly brushing over the detail on the model.
When the effect becomes less noticeable, I reload and rub the brush again.
If I happen to hit an area too strongly, I can reduce the harshness by dry brushing with the base color (this minor fix-up does not need to have retarder added – just use the base coat paint as it is).
Here is the smokebox end of an NYC J3A Hudson prior to dry brushing. Note there is airbrush weathering around the stack, some powder added around the sand filler caps, and black oil wash on the bright boiler handrails:
Here is the same area after grey dry brushing; the raised details are slightly more prominant:
There doesn’t appear to be much of a difference and this is good dry brushing. This subtle effect, however, is picked up by the eye – the detail is enhanced.
For me, this makes a worthwhile difference.