This is a rather advanced technique, and isn't often needed or useful. I tend to use it in cases where an object can be better vertex manipulated if it's one brush. Remember that the resulting object still has to be a valid convex solid. This includes not having two faces lie on the same plane. For the tutorial I made an odd conical shape out of three cylinders and a spike, don't ask what it is as I have no idea. First, copy the brushes to be merged to a new document. This will both ensure the safety of your original VMF, and make finding things easier. My object is made of four pieces, as you can see, and we need to differentiate between the inside faces that touch each other and the outside faces we see so we know what to remove. How many of each you have will determine which ones to change, I have 33 outside and 6 inside, so I'll change the insides. Material doesn't matter, for the tutorial I'm using nodraw on the faces to be removed. (brushes expanded to show what I did, keep them together for the next part) Save the map, then open it in a text editor. If you've never done this before it can look complicated, but it's easy to follow once you learn to recognize pieces. First we want to locate and remove all the inside faces, and we do so with the help of the material property each face block has in the VMF. Here I've found a side with nodraw on it, so I know this is one I wanted to remove. Select the whole block, as I have in the picture, and delete it. Continue doing this throughout the whole document, use the find function to search for the material you used on the inside faces to ensure you don't miss any. Right now we have a bunch of invalid brushes, since we removed some of their faces. So now we need to do the actual merging of the brushes. To do this we will locate and remove every start and end of brush data, except for the very first and very last. Save the document, open it in hammer, and you should have a single brush to copy back into your original map.