Not sure I can answer your question any better than has already been done by guys with a lot more knowledge on the subject....... but here's my 2c worth.
I have not tied a lot of dubbing brush wire bodies, the ones I do usually (like Ruard) get a thorax to help disguise tie in and tie off points.
I like to use a small thickness working thread, like Sheer #14 do I do not add too much additional bulk to the body. You do have to be careful with fine threads, you know how if you brush them over a sharp hook point and they do not sever until about three wraps on? Well they will do the same when wrapped over scissor cut wire.
Ruard sent over (with the last swap set) one of his brushes and the materials needed to tie "his" fly. I just completed that challenge.
I did not pay enough attention to Ruards master fly, I did not add the thorax and relied on the wire brush to form the entire body. I tied the hackle by the stem (after stripping away maybe half the material of Ruards already prepared feather), then tied in the wire brush directly behind the hackle tie in. Making sure the wire sits on top of the hook shank I wrapped back over the bare section to the end of the fly and back forward to the tie off position behind the hackle. Wound the brush forward, cut the wire, wrapped over and severed the thread twice
. Got the cut ends covered, wrapped the hackle one turn, bought the thread through and whip finished.With the small thread the tie in/tie off points are not too bulky and are somewhat hidden behind the large barbs of the Mallard hackle.
I had enough of the brush remaining to use for most of another fly, so tied another up on a Kamasan B420 Sedge hook #12.
This one I did a little different. Because the section of brush was short, tie in point was at the back of the thorax. I untwisted the wires a little to get two strands and tied in with one strand on either side of the hook shank. Thread wrapped over the wires to the body end, then forward to the tie off, wrapped the brush back and secured in. Dubbed the thread with some Aussie possum, wound a thorax to cover the tie in/tie off, wrapped a hackle and tied off.
I believe with a wire brush you need to tie in either right behind the hackle or behind a thorax if one is planned. Tie in at the butt and there will be a "hump" as the wire wraps back over the tie in point back to just your thread base....... unless you build your thread base up to match the height of the wire tie in.
I also believe when using wire brushes it pays to think on hook choice a little. The wire does not compress or flatten like silk bodies can and they carry a bit more bulk into the build, therefore I believe careful consideration should be given to either hook gape, or the position in which you choose the fly body to end.
Just my casual observations on a method I never claimed to be an expert in...... take from it what you will.
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