I guess how much or how little to polish the horn depends on what you intend to do with it. If you're just doing "primitive" artwork, I'd agree with Mike that 400 to 600 grit is fine. I polish mine up to 12000 grit before I start working on them.
As far a laying out the image first...I use a workable fixative (you can buy it from an art store) to cover the design after I've drawn it on. I tried hair spray but it rubbed off too easily and got too sticky simply from the warmth of my had resting on it, so I switched. I still use the technique for smaller areas like bands around the base and throat.
Unfortunately for larger areas that require longer periods to do, drawing the design on was an exercise in futility. Why? Because I ink constantly while I'm working and since the etcher's ink I use is very mildly abrasive, the fixative gets rubbed off quickly and with it goes the image and then i spend way too much time redrawing the image. Every time you have to redraw it it changes a little and it is time consuming to have to constantly stop engraving and redraw the image. So now I use a "stencil" technique to outline the image and then simply fill in the outline.
My advice is try a bunch of different things until you find one that works to your satisfaction and then stay with that technique and refine it until a better idea comes your way. I'm not an expert, but I've done a little bit of "scratching" and I'm more than willing to share what I've learned. Feel free to contact me if you want to chat.
Good luck!
Mark
